Seeding is OK, but Tixati times out all connection attempts to others seeders.

Haven't noticed it. Tested a random torrent, it downloads just fine.

There seem to be problems with connecting to local I2PSnark destination running on the same node, but it works with other peers.

What kind of I2P node do you use? Is it on the same computer or virtual machine, or accessed over the network?

Yes, some small torrent works, no success with larger ones.
Try this hash: (Hash removed by Mod and sent to Devs)
I2PSnark Standalone has 3 seeders, 7 leechers working
opentracker within tixati reports the same
Tixati only connects to the 7 leechers
Running i2pd without problems with torrenting for years