1. If you receive e.g. during a quicksend, matching or a pdf generation an error like
"Bad Request Invalid field: "xyz_MembershipStatus__c"
This is caused by a field which formerly belonged to a installed package and was deprecated in the meanwhile.
Please search the field and "delete" it, after you have checked, that you do not use this for relevant information anymore.
2. If you notice that it is a field you recently renamed or deleted and you get an error e.g. on Matching or when trying to create a PDF or Quicksend email like this:
Please clear your cache and try reloading the page where you got the error. The renamed or deleted field was still locally stored in your browser's cache and hence used in your query. Once you clear your cache, the fields and values will be freshly aggregated from the database and the error should disappear.