Synology / POP3 for only the main adres does not work

[R2FM]

As a Synology user, I upgraded from DS416 to DS1522+. When converting, the configuration crashed and I reconfigured it on the 1522. In retrospect, most things fell back into place. The last major step was the mail, which still worked fine on 416, but does not want to work on 1522 for the existing main (!) POP3 address. Very strange: The 4 other accounts within the family work without any problem. I found the following 2 tips;

• Tip 1 was to delete and re-enter user. Unfortunately no success.
  
• Tip 2 to do that again after restarting the server.

When entering user again in the POP3 menu, the message from Roundcube was: "This address already exists", while I had really deleted it and the POP3 addresses were empty; the address does not exist for other users either.
On the webmail I can see that there is enough mail to retrieve; I don't forget about passwords either. Seems like I'm overlooking something somewhere. Does anyone have a tip? Thank you in advance ! Regards, Ron (NL)

[Dmitry42]

I can tell only one tip - try reinstall sinology mail server.

As I know Synology used system users and mail mailbox stored in 'homes' folder. So when you reinstall mail server - users and their folders will be on the same place.

PS I think you can backup 'homes'