Permalinks broken regularly

On one of my sites the permalinks are broken regularly. After saving them again it’s all fine but a few days later the permalinks result in not found and I have to save them again.

There are no errors in the log files and I also deactivated all plugins which might impact rewrites/permalinks.

Due to the fact that it happens over time it’s hard to see if maybe a plugin is causing the issue.

Any idea?

I once had a problem like this. On my site was a malware and it edited a .htaccess file. Check by FTP that your .htaccess file is not changed. If yes, scan your site by any online malware scanner or any security WordPress plugin.