Serverside only. Client side, there are no issues.

forge-1.12.2-14.23.5.2838
spongeforge-1.12.2-2838-7.1.7-RC3924

Error snippet:

[15:46:32] [main/INFO] [Sponge]: / Error Type : org.spongepowered.asm.mixin.transformer.throwables.MixinTargetAlreadyLoadedException /
[15:46:32] [main/INFO] [Sponge]: / Caused by : Unknown /
[15:46:32] [main/INFO] [Sponge]: / Message : Critical problem: mixins.common.api.json:mcp.block.properties.IPropertyMixin_API target net.minecraft.block.properties.IProperty was already transformed. /

Full log:
debug.log

We've got a lot of mods (127) and I don't have a list on-hand. Hopefully the log file should provide enough information though, sorry if not.