There's obviously no check for too long filenames:
in my case processing works if there are no more than 4 steps. If a 5th step is added the job will fail:
digikam.general: Action Thread run  1  new jobs
digikam.general: Tool : index=  1  :: name=  "Resize"  :: group=  4  :: wurl=  QUrl("file:///storage/photos/")
digikam.general: path:  "/storage/photos/BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg"
digikam.general: Tool:        "Resize"
digikam.general: Input url:   QUrl("file:///storage/photos/IMG_20170616_151042.jpg")
digikam.general: Output url:  QUrl("file:///storage/photos/BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg")
digikam.dimg: "/storage/photos/IMG_20170616_151042.jpg"  : JPEG file identified
digikam.metaengine: Loading image history  ""
digikam.metaengine: DateTime => Exif.Photo.DateTimeOriginal =>  QDateTime(2017-06-16 15:10:42.000 CEST Qt::TimeSpec(LocalTime))
digikam.general: Detected change, triggering rescan of "/storage/photos//"
digikam.database: Starting scan!
digikam.metaengine: Exif color-space tag is sRGB. Using default sRGB ICC profile.
digikam.general: Tool : index=  2  :: name=  "AssignTemplate"  :: group=  8  :: wurl=  QUrl("file:///storage/photos/")
digikam.general: path:  "/storage/photos/BatchTool-T29661-BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg"
digikam.general: Tool:        "Apply Metadata Template"
digikam.general: Input url:   QUrl("file:///storage/photos/BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg")
digikam.general: Output url:  QUrl("file:///storage/photos/BatchTool-T29661-BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg")
digikam.general: Detected change, triggering rescan of "/storage/photos//"
[...]
digikam.general: Tool : index=  3  :: name=  "WaterMark"  :: group=  5  :: wurl=  QUrl("file:///storage/photos/")
digikam.general: path:  "/storage/photos/BatchTool-M29661-BatchTool-T29661-BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg"
digikam.general: Tool:        "Add Watermark"
digikam.general: Input url:   QUrl("file:///storage/photos/BatchTool-T29661-BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg")
digikam.general: Output url:  QUrl("file:///storage/photos/BatchTool-M29661-BatchTool-T29661-BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg")
digikam.general: Detected change, triggering rescan of "/storage/photos//"
digikam.database: No itemShortInfo could be retrieved from the database for image "BatchTool-T29661-BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg"
digikam.metaengine: Orientation => Exif.Image.Orientation =>  1
digikam.database: Starting scan!
digikam.database: No itemShortInfo could be retrieved from the database for image "BatchTool-T29661-BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg"
digikam.metaengine: Orientation => Exif.Image.Orientation =>  1
digikam.general: Tool : index=  4  :: name=  "WaterMark"  :: group=  5  :: wurl=  QUrl("file:///storage/photos/")
digikam.general: path:  ""
digikam.general: Tool:        "Add Watermark"
digikam.general: Input url:   QUrl("file:///storage/photos/BatchTool-M29661-BatchTool-T29661-BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg")
digikam.general: Output url:  QUrl("")
digikam.database: No itemShortInfo could be retrieved from the database for image "BatchTool-M29661-BatchTool-T29661-BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg

I assume this is caused by a too long filename:
~> touch Batchtool-A12345-BatchTool-M29661-BatchTool-T29661-BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg
touch: cannot touch 'Batchtool-A12345-BatchTool-M29661-BatchTool-T29661-BatchTool-J29661-IMG_20170616_151042.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg.digikamtempfile.jpg': File name too long

In this case filesystem is a NFS mount (v4, ext4 on host)

if you need any other data, i can try to deliver.. 


~> zypper if digikam
Loading repository data...
Reading installed packages...


Information for package digikam:
--------------------------------
Repository     : openSUSEKDEExtra                  
Name           : digikam                           
Version        : 5.6.0-176.1                       
Arch           : x86_64                            
Vendor         : obs://build.opensuse.org/KDE:Extra
Installed Size : 88.9 MiB                          
Installed      : Yes                               
Status         : up-to-date                        
Source package : digikam-5.6.0-176.1.src           
Summary        : A KDE Photo Manager               
Description    :                                   
    digiKam is a simple digital photo management application for KDE, which
    allows you to import and organize your digital photos easily. The
    photos can be organized in albums, which can be sorted chronologically,
    by directory layout, or by custom collections. An easy-to-use interface
    that enables you to connect to your camera and preview, download, or
    delete your images, is provided.