Hi Moostic,
Sounds like you need the 4gb patch. Could be that running GWX and U-boats compilation together is just too much for SH3 to handle without it.
4gb patch included with this, one of the greatest:
http://www.subsim.com/radioroom/showthread.php?t=174225
Or, by itself:
https://ntcore.com/?page_id=371