crash report - https://hatebin.com/msbapjwlke
latest.log - https://hatebin.com/mftbkarvie

This probably has to do with Java 15, not the Fabric version.

(Workaround for now: Disable general.taggable_players)

Well I don't know how to disable general.taggable_players but I used Fabric Zero mod and it worked. Can you fix it to work with Java 15 or this workaround is the only possibility?

I'll fix it soon, but in general I'd recommend you use the latest Java LTS rather than bleeding. Fabrication works fine on 8 and 11. It seems Oracle broke a (very old) reflection trick that allows you to modify final fields in 15 and I'll probably have to use an access widener instead of this trick.

You can disable general.taggable_players in fabrication.ini.

[general]
; >>snip<<
; Allows players to be tagged by ops with /fabrication tag.
; Recommended to leave enabled unless there's an extremely unlikely
; incompatibility with another mod.
taggable_players=true