I use KeePass with mono. I had been using mono 2.10.9 with KeePass, it's latest versin 2.27. My version of libgdiplus is version 2.10.9. I am running on Slackware64-current.
I upgraded mono 3.2.3 and encounter no problems in the compile or running KeePass. I then decided to upgrade to mono 3.6.0 using the source tarball. It works with KeePass. That said, I did run in to a few problems. This only applies to the source tarball you can get from http://download.mono-project.com/sources/mono/ and not from github.
If you have an older version of mono, and I did, it must be removed before compiling mono as it will try to use the older version during the build process. Removing the older version of mono solves this.
The SlackBuild script for mono on SBo needs a slight modification, aside from changing the VERSION to match the mono version. Find README in the docs part of the script near the end and change it to README.md
After a successful run with the modified mono.SlackBuild and running keepass I encountered a similar, yet different error than yours, basically it could not find /usr/lib/libgdiplus.so.
I'm on a x86_64 system and libgdiplus is located in /usr/lib64/. The quick fix was to create a simlink
Code:
ln -s /usr/lib64/libgdiplus.so /usr/lib/libgdiplus.so
KeePass is installed using the SlackBuild from SBo.
By the way, on a standard installation of Slackware the only requirement for mono is libgdiplus.