issues with time machine

[Wder]

I have an issue with time machine, which I hope someone can help with. I have an old MacBook Air running El Capitan, and on that machine time machine is running without issues on my Synology. I recently got a MacBook Pro on which I am running Catalina. I managed to got time machine to work on it, but every time there is an update I have an issue where I get a message that the volume for time machine on my Synology is not available. I can acces the volume in finder without issues however. Manually deleting the old backup copy and then restarting time machine seems to work out fine, but of course I do loose my older backups every time this way. Any suggestions ? Thanks a lot in advance !