Yea, I have seen the same thing with 5.5.3 with BaseHead.
Nuendo crashes directly after an auto-save when “Follow Session Folder” is ON in BaseHead.
That is one point when Nuendo will pass to BH what project is open or active currently and it’s making N5.5.3 explode right now.
I have an email out to once of the developers now to see what was changed in 5.5.3 and make them aware of it.
For now, leave that option OFF in BaseHead to avoid Nuendo going down while working.
p.s. If you want to grab the session folder quickly still you can tick in ON and OFF quickly without a problem.