(Solved already, I'm writing this for the next guy)
I was running git daemon on one computer and tried synchronizing with another.
On computer A, I ran:
git daemon --reuseaddr --base-path=. --export-all --verbose
On computer B, I ran:
git clone git://computerA/.git source # worked
cd source
git pull # worked
git push # failed with "fatal: The remote end hung up unexpectedly"
On computer A, the daemon output is:
[5596] Connection from 127.0.0.1:2476
[5596] Extended attributes (16 bytes) exist <host=localhost>
[5596] Request receive-pack for '/.git'
[5596] 'receive-pack': service not enabled for './.git'
[5444] [5596] Disconnected (with error)
I'm going to post the soultion I found. If you have a more complete answer, please go ahead and add it.