IBM released a new Hotfix for Domino 8.5 (IF2). This fix solved the following problems:
A regression in Lotus Domino 8.5 with user rename and user delete fails to update group membership, except in the case where the renamed or deleted user is also listed in the LocalAdmin field for the group. This issue has been reported to Quality Engineering as SPR#JPAI7MFS69. A fix is available for this issue.
In Lotus Domino 8.5, you will see the IMAP task crash your Domino Server in certain instances. This issue has been reported to Quality Engineering as SPR #JCHS7NNM56. A fix is available for this issue.
These fixes are included in 85IF2 which includes SPR#JPAI7MFS69 and SPR #JCHS7NNM56.
The fix can be downloaded from the IBM FixCentral.
You can find more infos regarding the fix on the IBM Website.