I had the same problem a week ago. and unable to replicate it on my test server. I found out that I had tunred on the debug plugin on and that it had reached a massive size due to some irrelevant warnings being dumped over and over again (at each SELECT, there were at least 20 lines added...). I deleted the error.log file and managed to get access again to the site. unfortunately, it was short lived since some pages continued to be blank. I ended up doing a complete reinstall of the system (files + DB)...
 
So if anyone has a solution to that I am also very interested.