thanks for your help. There are 2 things I want to add:
1.) We are using this version because it is a preview-container and we want to evaluate the software for enterprise use, but on premise. I can upgrade, but I already did to this version, which was the latest at time of the update.
2.) We tested and the messages arent sent at the time shown in the chat. We tested with sending the current timestamps of the computer clocks as message text. A collegue is sitting next to me in the same room and his message is timestamped several minutes in the past, at a time he didnt wrote, yet. The next message, send seconds after, is in correct order and message text (time) is in sync with Timestamp. Sometimes from 3 short messages send in 10 seconds apart, 2 are correct, one is not (all from the same client). And for the record, the messages are showing up immediately.
Despite this problem, we have freezes and white screens on several Windows 10 Clients with Client 4.2 and problems with Empty Search Window and links to already deleted messages crashing the client. I think the software isnt mature enough for our needs. I will try to update the Server to newest software and check the logs.