Hello, identity conflict usually means:

1. Your Steam Identity is already linked to some other BI Account (probably personal)
2. BI Account which you are trying to link is already linked to another Steam Identity

Origin of your issue comes actually with Steam integration with Workbench which solve different subset of problems.
Only one person is able to upload it because specific Steam Identity is owner of that addon unless Steam account is also shared.
Logout itself in Workbench will not unlink your account so you have to unlink it ingame.
This is something we are aware of and is on our list of things to solve for future, for now only solution is to share Steam account and properly link/ unlink.
Can you send us GUID of your addon? We could look for which identity it is linked to and possibly we could change it to another steam identity if you need.