Adding github repo: Error: Unexpected encoding - UTF-8 or ASCII was expected

[LinusSieber]

Trying to add a repo from github on repman.io results in:
Synchronization error
4 seconds ago

Error: Unexpected encoding - UTF-8 or ASCII was expected
Logs:
Reading composer.json of rent2buy/teams-workflow (v2.0)
Importing tag v2.0 (2.0.0.0)
Reading composer.json of rent2buy/teams-workflow (main)
Importing branch main (dev-main)

I verified that composer.json is indeed UTF-8 encoding with Unix line separators.

What to do?