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When this happened to us it was because the skin being used had somehow become linked to a non-existant one.
Could something not have been put in the upcp update list to fix this issue as it appears?
Update: Cpanel was set to use a theme which didn't exist.
Symlinking another theme to place it was looking fixed the issue up.
Am also seeing this issue on a server...
Fix in other thread has no effect (2086 doesn't work either)