Second SQL Node does not connect when 1st is online

Our mysql cluster with two nodes is online for several years now and started to cause issues two days ago.

The issue is that the second sql node changes from connected to disconnected state all the time and the sql server is not reachable if it does that.

When I shut down the first node though, the sql node stays connected and everything is fine. Once I set the first node online again the 2nd node behaves as before.

Server logs after I turned on the sql node of the other server:

NDB Schema dist: Data node: 1 failed, subscriber bitmask 00
NDB Schema dist: cluster failure at epoch 74253579/0.
NDB Binlog: cluster failure for .\mysql\ndb_apply_status at epoch 74253579/0.
NDB Binlog: Restarting
NDB Binlog: Stopping...
NDB Binlog: Setting up
NDB Binlog: Created schema Ndb object, reference: 0x80030033, name: 'Ndb Binlog schema change monitoring'
NDB Binlog: Created injector Ndb object, reference: 0x80040033, name: 'Ndb Binlog data change monitoring'
NDB Binlog: Setup completed
NDB Binlog: Wait for server start completed
NDB Binlog: Check for incidents
NDB Binlog: Wait for cluster to start
NDB Binlog: DISCOVER TABLE Event: REPL$mysql/ndb_schema
table '.\kunden\kundendaten_licencedata' opened read only
NDB Binlog: ndbevent->execute failed for REPL$mysql/ndb_schema; 1427 Api node died, when SUB_START_REQ reached node
NDB Binlog:FAILED CREATE (DISCOVER) EVENT OPERATIONS Event: REPL$mysql/ndb_schema
NDB: Creating mysql.ndb_schema
NDB: Flushing mysql.ndb_schema
NDB Binlog: DISCOVER TABLE Event: REPL$mysql/ndb_schema

To ensure it is not the firewall I turned them off on both servers. I also tried to reinstall the cluster on the 2nd server, didn't help. Out of ideas now.