Sometimes I get that error when I'm approaching my memory limit or the memory has got fragmented from a long session.
It doesn't help none when trying to save a project, but if approaching memory limits is the cause, firing up Sonar from a fresh reboot will minimize the likelyhood of it happening during a memory intensive session.
It can also explain why a project will seem OK one minute and not the next as Windows memory management is allocating and freeing memory handles automatically in the background. And whilst Sonar is pretty good at assessing it's own memory requirements some (most notably) VSTi's can cause surprises that leave Sonar baffled (hence that particular vague message) as the plug hasn't reported back in a way that Sonar can understand, what the problem was.
Ergo ***t happens and starting from a clean slate can mitigate it somewhat.
Save revisions and save often, we all know that, but I don't do it either...
edit: I meant to say freezing some soft-synths if possible will help there too.