Problem solved!



You were right it was about certificates.



For some reason the certificates in the key and trust stores were messed up. I read somewhere that this was due to Websphere not cleaning up correctly after automatically issuing new certificates.



Apparently this is fixed in 6.1.0.15.



What I did was:

cleaned up so there was only one certificate left in the stores,

made sure it matched on the alias (not sure this is needed)

restarted



Now the node was synchronized



I worry a bit about side effects that might occur, so I'll watch it a bit closer the next couple of days...



Thanks for the all the help.