Synchronisation stopped?

I have my nextcloud box installed and three clients connected. All worked fairly well, but since two days there is no file synchronisation any more. Why?
The client software is active and tells me, that there is a connection with my nextcloud box. And that the last synchronisation was succesfully. But the dates and sizes of my files on my local disk and in the nextcloud box are different! And there is no activity to synchronize.
Any help?

So the files shown in the web-interface are different of what the client syncs? Can you check your logfiles for errors? Which versions are you using (server/client)?

Thanks for your answer.
Fortunately the problem was solved in the meantime