Found the problem, most of it anyway. The issue was how R was compiled. Doing a make/make install of R's src/nmath/standalone fixed the cantor build. However, after launching cantor and selecting R as the backend, cantor never finishes its initialization session. From the cli I see this;

dveatch@sidney ~ $ cantor
Could not find drkonqi at /usr/lib64/libexec/drkonqi
kf5.kservice.sycoca: Trying to open ksycoca from "/home/dveatch/.cache/ksycoca5"
Creating ScilabBackend
Creating PythonBackend
Creating NullBackend
Creating SageBackend
Creating RBackend
Creating MaximaBackend
Created a CantorPart
Backend  "R"  offers extensions:  ("ScriptExtension", "AdvancedPlotExtension")
Spawning a new R session
()
()
()
()
()
()
()
()
()
()
()
()
(QKeySequence("Ctrl+Shift+S") ) 
(QKeySequence("Ctrl+S") ) 
()
(QKeySequence("Ctrl+Z") ) 
(QKeySequence("Ctrl+Shift+Z") ) 
(QKeySequence("Ctrl+X") ,  QKeySequence("Shift+Del") ) 
(QKeySequence("Ctrl+C") ,  QKeySequence("Ctrl+Ins") ) 
(QKeySequence("Ctrl+V") ,  QKeySequence("Shift+Ins") ) 
(QKeySequence("Ctrl+F") ) 
(QKeySequence("Ctrl+R") ) 
(QKeySequence("F3") ) 
(QKeySequence("Shift+F3") ) 
()
(QKeySequence("Ctrl+P") ) 
(QKeySequence("Ctrl++") ,  QKeySequence("Ctrl+=") ) 
(QKeySequence("Ctrl+-") ) 
()
()
()
()
()
()
()
(QKeySequence("Shift+Return") ) 
(QKeySequence("Ctrl+Return") ) 
()
()
()
()
(QKeySequence("Shift+Del") ) 
()
()
()
()
loading assistants...
found service "Eigenvectors"
created it
plugin  "Eigenvectors"  requires  ("LinearAlgebraExtension", "HistoryExtension")
plugin  "Eigenvectors"  is   not   supported by  "R"
found service "Invert Matrix"
created it
plugin  "Invert Matrix"  requires  ("LinearAlgebraExtension", "HistoryExtension")
plugin  "Invert Matrix"  is   not   supported by  "R"
found service "Differentiate"
created it
plugin  "Differentiate"  requires  ("CalculusExtension")
plugin  "Differentiate"  is   not   supported by  "R"
found service "Eigenvalues"
created it
plugin  "Eigenvalues"  requires  ("LinearAlgebraExtension", "HistoryExtension")
plugin  "Eigenvalues"  is   not   supported by  "R"
found service "Import Package"
created it
plugin  "Import Package"  requires  ("PackagingExtension")
plugin  "Import Package"  is   not   supported by  "R"
found service "RunScript"
created it
plugin  "RunScript"  requires  ("ScriptExtension")
plugin  "RunScript"  is    supported by  "R"
KXMLGUI file found at deprecated location ("/usr/share/cantor/cantor_runscript_assistant.rc") -- please use ${KXMLGUI_INSTALL_DIR} to install this file instead.
found service "Integrate"
created it
plugin  "Integrate"  requires  ("CalculusExtension")
plugin  "Integrate"  is   not   supported by  "R"
found service "Solve"
created it
plugin  "Solve"  requires  ("CASExtension")
plugin  "Solve"  is   not   supported by  "R"
found service "Plot2d"
created it
plugin  "Plot2d"  requires  ("PlotExtension")
plugin  "Plot2d"  is   not   supported by  "R"
found service "Plot3d"
created it
plugin  "Plot3d"  requires  ("PlotExtension")
plugin  "Plot3d"  is   not   supported by  "R"
found service "QalculatePlot"
created it
plugin  "QalculatePlot"  requires  ("QalculatePlotExtension")
plugin  "QalculatePlot"  is   not   supported by  "R"
found service "Create Matrix"
created it
plugin  "Create Matrix"  requires  ("LinearAlgebraExtension")
plugin  "Create Matrix"  is   not   supported by  "R"
found service "AdvancedPlot"
created it
plugin  "AdvancedPlot"  requires  ("AdvancedPlotExtension")
plugin  "AdvancedPlot"  is    supported by  "R"
KXMLGUI file found at deprecated location ("/usr/share/cantor/cantor_advancedplot_assistant.rc") -- please use ${KXMLGUI_INSTALL_DIR} to install this file instead.
loading panel plugins for session of type  "R"
found service "Variable Manager"
created it
plugin  "Variable Manager"  requires  ("VariableManagementExtension")
plugin  "Variable Manager"  is   not   supported
found service "Help"
created it
plugin  "Help"  requires  ()
Requesting capabilities of RSession
plugin  "Help"  is    supported
setting session to  0xad6510
Requesting capabilities of RSession
QObject::disconnect: Unexpected null parameter
Shortcut for action  "format_text_bold" "&Bold" set with QAction::setShortcut()! Use KActionCollection::setDefaultShortcut(s) instead.
Shortcut for action  "format_text_italic" "&Italic" set with QAction::setShortcut()! Use KActionCollection::setDefaultShortcut(s) instead.
Shortcut for action  "format_text_underline" "&Underline" set with QAction::setShortcut()! Use KActionCollection::setDefaultShortcut(s) instead.
Shortcut for action  "format_text_strikeout" "&Strike Out" set with QAction::setShortcut()! Use KActionCollection::setDefaultShortcut(s) instead.
Shortcut for action  "evaluate_current" "Evaluate Entry" set with QAction::setShortcut()! Use KActionCollection::setDefaultShortcut(s) instead.
Shortcut for action  "insert_command_entry" "Insert Command Entry" set with QAction::setShortcut()! Use KActionCollection::setDefaultShortcut(s) instead.
Shortcut for action  "remove_current" "Remove current Entry" set with QAction::setShortcut()! Use KActionCollection::setDefaultShortcut(s) instead.
Shortcut for action  "show_completion" "Show Completion" set with QAction::setShortcut()! Use KActionCollection::setDefaultShortcut(s) instead.
updating panels
adding panel for  "Help"
login
wsStatusChange 1
Could not find drkonqi at /usr/lib64/libexec/drkonqi
storing plots at  "/tmpcantor/rserver-24610"

R version 3.2.0 (2015-04-16) -- "Full of Ingredients"
Copyright (C) 2015 The R Foundation for Statistical Computing
Platform: x86_64-pc-linux-gnu (64-bit)

R is free software and comes with ABSOLUTELY NO WARRANTY.
You are welcome to redistribute it under certain conditions.
Type 'license()' or 'licence()' for distribution details.

  Natural language support but running in an English locale

R is a collaborative project with many contributors.
Type 'contributors()' for more information and
'citation()' on how to cite R or R packages in publications.

Type 'demo()' for some demos, 'help()' for on-line help, or
'help.start()' for an HTML browser interface to help.
Type 'q()' to quit R.

setting up callbacks
integrating plots
running command  "png(filename="/tmpcantor/rserver-24610/Rplot0.png", width = 480, height = 480, units = "px")"
PARSING  "png(filename="/tmpcantor/rserver-24610/Rplot0.png", width = 480, height = 480, units = "px")"  went OK
readConsole:  Selection: 


All the other backends initialize fine.

The code was refactored, compiles without problem with several used compilers and distributions. Please check again and re-open this ticket if you still have this problem.