Answer for @helgeo
Yes, what is wrong is you still have the "exceeding memory" problem, even with your increase to 512M, it's trying to use 536M, as per the error. Hence, if you convert bytes to MB you get:
536870912 bytes = 536.87M
Not sure what to suggest, other than increasing the memory once again to say, 768M or something more than 512M.
However I am guessing that PDF conversion of the form submission details is resource intensive and consumes a lot of memory, as you have already seen it doing.
I believe this would be better answered by Appnitro and suggest you submit a help ticket to them through your Member area (login required, current support subscription required).
If you can't raise the memory limit any higher, then Appnitro will need to investigate if it is the PDF conversion that's causing the trouble as you suspect.