In the beginning of the iptables configuration I’ve changed the lines
-A INPUT -d 127.0.0.0/8 -j REJECT
# last line different from debian wiki and taken from http://www.rootathome.de/firewall-mit-iptables-unter-debian-einrichten/
with this one:
-A INPUT ! -i lo -d 127.0.0.0/8 -j REJECT
The current one is the original one from the Debian wiki. I had to use the former one in Seafile v4 because otherwise I couldn’t sync. Nobody could tell me what is the reason for. Recently I found out that the Debian wiki version works fine with Seafile v5 now.