I installed Redmine on my Raspberry Pi 2. I tried to install some plugins, but almost all plugin causes Error 500.
Redmine version 2.5.2.devel
Ruby version 2.1.5-p273 (2014-11-13) [arm-linux-gnueabihf]
Rails version 4.1.8
I only tried compatible plugins and I can't find the log files (I searched through the entire redmine directory)
What goes wrong? And should I upgrade to V 3.x?
You are running an old version of Redmine. You should install the new version which is 3.3.2. After installing the new version of Redmine you should have a log directory in the Redmine installation folder. Inside the log folder there should be a production.log file which has the log information.