It's strange that you get no trace. You may have to find your PHP error log and look there. Without the trace, we can't see the source of the error which will make it very difficult to diagnose. What operating system / Linux distro (or whatever) is this - maybe we can tell you where to look for logs?
I've not heard of the LearnR theme but it should be fine - there's not supposed to be any breaking changes in minor point upgrades.