I noticed a strange behavior in a M2 System, I think this is related to a 3rd party module. All layouts which should be 1column layouts are 2 column layouts actually so something overwrites the default behavior. In conclusion, they got the wrong body class page-layout-admin-2columns-left instead of page-layout-admin-1column. I followed this thread How can I debug layout file processing in Magento 2? to debug this, but can't see anything wrong. So my questions are, how to debug this? Any hints are helpful.