use the following Powershell script:
Disable-SPFeature -identity enhancedtheming -url http://sharepointsite/sitename
or:
themes can be found at: http://[yoursharepointsite]/[yoursitename]/_catalogs/theme/Forms/AllItems.aspx
you could clear out this list. (always backup them)
Komt deze foutmelding vaak, soms of altijd voor?