Problems synchronizing folders with Outlook classic and icloud

Outlook stopped synchronizing the folders with Mail and icloud. Some of the subfolders in Mail do synchronize and show in Outlook but some do not show. The error-no shown is  [800CCC0E-0-0-940]. The folders lost seem to be folders that have names starting with letters down in the alphabet. Outlook organizes the subfolders in the inbox in alphabetical order and I get them up to the folder-names starting with K. I tried to fix it by replacing the xx.ost - file that outlook uses to store data with a backup. The result was that the folders showed until it started synchronizing - and then they disappeared.