Diagnosis
The receipt you want to delete is a document from the document bufferfor credit-card clearing.
Receipts of this kind can not be deleted in a trip, but only in theselection popup in the document buffer.
Since access to the document buffer is momentarily blocked, thisreceipt can not yet be sent back to the document buffer.
A possible cause for the blocking could be a current import ofcredit-card accounting data by your system administrator.

Procedure
Wait a few minutes and then try again.