please, open a discussion thread on this workshop page, or Uboat Discord #ube-general-chat channel

some of the following is needed:
output log
(assuming new game)
(as stated no other mods)
while loading the game (it sounds like the initial load + building of UBE)
please check the following directory for Uboat expanded .dat files being created during launch:
C:\\users\<user>\appdata\locallow\DeepWaterStudio\UBOAT\Data Sheets

the following files should be created

"[WIP B129] U-Boat Expanded - CharacterClasses.dat"
"[WIP B129] U-Boat Expanded - Crew.dat"
"[WIP B129] U-Boat Expanded - Entities.dat"
"[WIP B129] U-Boat Expanded - General.dat"
"[WIP B129] U-Boat Expanded - Graphics.dat"
"[WIP B129] U-Boat Expanded - Locales.dat"
"[WIP B129] U-Boat Expanded - Sandbox.dat"

this one time process can take up to 5 minutes (depending on machine)

also note there are workarounds for users using WINE (linux)