I've had the same problem in the last version and as I've started to do longer renderings to get more details of zoomed in sections this really becomes a problem:
When I safe pictures after longer rendertimes (like 12hours+) the chances of crashing while export seems to rise very much. After 24hours I can be pretty sure that as soon as I hit 'safe' it will crash.
The result is a half-safed png as attached (downsized, was 7680² resolution), it seems to crash at different points. And all the days of rendering are lost.
I don't kjnow what more info I could provide you with but I really hope you find a solution to this. maybe a different way of saving the progress before exporting, so that even if it crashes, not all progress is lost and one can continue at the last savepoint?
-happens on cpu as well as gpu
-not sure but I feel it happens much more often after I switched filter type from linear/square to log&log10
-large rendersizes of over 7680*7680
-core i7-3770, 32gb ram, windows 10 64 bit
-Sapphire Vapor-X Radeon R9 290 Tri-X OC, 4096 MB GDDR5
(besides that: this is currently my absolute favourit program, running 24/7, chewing out the most impressive pictures I've ever seen of the mandelbrot-set - and I've seen a lot! will share, when I'm ready
)