I had the same exact problem. This is the solution.
Slack64 14.1 (Alienbob multilib), but there is no need for compat32 libgdiplus.
After compling/installing libgdiplus sbopkg/slackbuilds.org be sure to run $ ldconfig -v and verify if there is libgdiplu.so in /usr/lib64 path. Then install mono using sbopkg/slackbuilds.org
trying to run KeePass gives an error at this moment because Mono is looking for libgdiplus.so in fixed path /usr/lib, totally ignoring LD_LIBRARY_PATH and ldconfig cache.
And this is solution:
so edit Your /etc/mono/config as follows:
# diff config.old config
< <dllmap dll="gdiplus" target="/usr/lib/libgdiplus.so" />
< <dllmap dll="gdiplus.dll" target="/usr/lib/libgdiplus.so" />
> <dllmap dll="gdiplus" target="libgdiplus.so" />
> <dllmap dll="gdiplus.dll" target="libgdiplus.so" />
IT should work right now
tested with original 2.15 from slackbuilds.org and 2.24