It happened to me that all my folders of an IMAP account (cyrus imap server) turned red. After playing with custom colors I found out that this was due to reaching the quota limit (600MB). I then increased my quota settings on the IMAP server to 700 MB, but the folders stay red. I tried akonadi restart, reboot, nothing helps. However, if I open the folder properties dialog for one folder and close it again, no matter if you use cancel or ok, the folder color turns into black again. But only this single folder, the rest stays red. Reproducible: Always
Created attachment 76938 [details] Screenshot of the problem
*** This bug has been marked as a duplicate of bug 311996 ***