I’ve tried different forge versions with the corresponding spongeforge versions but still end up with this same issue on a FTB Direwolf 2.5.0 server. This crash was using Forge 2825 with the corresponding SpongeForge. Any insights as to why this is occuring?
Have you done the foamfix fix? Also you can try getting Sponge to load first by renaming it to aaaSpongeForge which can fix a few issues.
FoamFix is not in this pack. Also, the naming change does not help unfortunately 
Check again. The crash suggests that foamfix-0.10.3-1.12.2.jar is loaded
Ah! I somehow overlooked that, cheers!