[x]):Gitea does not allow migrating/mirroring repos with an empty master branch or no master branch.
Ideally the repo to be cloned should always be cloned as is. The default branch set for the repo must also be mirrored and be used by Gitea.
I tried cloning bcoin-org's lcoin repo from GitHub without success. https://github.com/bcoin-org/lcoin.git

This no longer seems to be an issue on master. I mirrored the repo in OP successfully to https://try.gitea.io/cez81/lcoin
Only 1 branch, litecoin. No master branch.
I try this local with v1.6.0-rc2 and also on https://try.gitea.io/repo/

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Seemed to be fixed at least in Gitea Version: 1.10.0+dev-188-g2d0b90c96
Please feel free to reopen it.
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This no longer seems to be an issue on master. I mirrored the repo in OP successfully to https://try.gitea.io/cez81/lcoin
Only 1 branch, litecoin. No master branch.