Not all Git submodules can be fetched

[th-otto]

Some git submodules fail do be fetched when doing a git clone --recurse-submodules. I.e. in C/Christian Zietz/uip-tools i get the error

fatal: remote error: upload-pack: not our ref 6f392786a147fd9e4a065c04bc7d74ce7c247353
fatal: the remote end hung up unexpectedly
Fetched in submodule path 'C/Christian Zietz/uip-tools', but it did not contain 6f392786a147fd9e4a065c04bc7d74ce7c247353. Direct fetching of that commit failed.

Same error also appears afterwards, when trying to do a git submodule update