Today I installed Connections 5.5 CR3 at customer site and followed the required steps here. I also followed the steps for creating the requested Dynacache for the Rich Text Editor:
As mentioned I created the Dynacache just for the RTE-Cluster and figured out, that this led to problems for all single cluster member.
So I removed the setting for the RTE-Cluster and added it for the complete cell:
After a restart of all cluster servers everything was fine again.