Seems to happen without correlation to fetching mail after a 30 to 180 minute run time. No problem after killing with broken window 'Force Quit button' (Linux Mint Mate 18.3, AMD 6 core)
version 3.13.2 was released over 4 years ago, there have been 10 releases since. Please upgrade to the latest release and, if you still have this problem, then leave a comment on this bug report.