khelpcenter in KDE4 cannot open docbooks from KF5 applications. It happens because it cannot find dtd/kdedbx45.dtd file in its search locations. Reproducible: Always
Did you try the Frameworks 5 version of khelpcenter? Please note that we are still shipping (by mistake, I think) both versions: kdelibs4-based as part of kde-runtime in KDE Applications and the KF5 version as part of Plasma 5 (but completely independent from Plasma, just released together).
Just to be clear: it's unlikely that khelpcenter/kdelibs4 is going to be fixed for this. khelpcenter/KF5 should be able to open both files (provided you install kdelibs4support; if it does not work, then it should be fixed.
khelpcenter/KF5 is able to display both kde4+kf5 based docbooks in Kubuntu 15.04 with Frameworks 5 + Plasma 5 From my point of view this report should be closed as Resolved/Invalid or Resolved/Downstream because distributions should install khelpcenter/KF5 for kf5 docbooks
Created attachment 93341 [details] Patch to add KF5 docbooks support to KDE4
KF5 khelpcenter works with both KDE4 and KF5 docbooks but there are some reasons why I (as ROSA's KDE maintainer) prefer to use old khelpcenter in KDE4: 1. ROSA uses /usr/share/doc/HTML/ path for KDE4 docbooks and /usr/share/doc5/HTML/ for KF5 docbooks. It's needed to avoid file conflicts. KDE4 khelpcenter uses first location as main search path and second location for fallback search. KF5 khelpcenter does the opposite, it prefers doc5 over doc. So it makes sense to use KDE4 khelpcenter for KDE4 and KF5 khelpcenter for Plasma5. 2. While KDE4 is our "main" official desktop, it's better to use as many native (KDE4) components as possible. KDE4 desktop shouldn't depend on KF5 packages. Meanwhile I used this patch to fix the issue downstream (attached a copy to make sure it's never lost): https://abf.rosalinux.ru/import/kdelibs4/blob/rosa2014.1/kdelibs-4.14.8-kf5-dtd.patch If some other distributions want to use it, make sure to have @DOCBOOKXML_CURRENTDTD_DIR@ defined as DTD 4.5 path.