Good day Simon,
The error is probably coming from your SQL Server but the source of the problem related to code which included in the stored procedure which you use spMFUpdateTable
Without seeing the code we cannot know what the course of the issue is.
As Tom said, you should contact the company which provide you the service that include this SP which is probably part of MFSQLConnector product:
https://lamininsolutions.atlassian.net/wiki/spaces/MFSQL/pages/31817730/spMFUpdatetable+Class+Table+Records
Note! the stored procedure sp_xml_removedocument is a build-in SP in the server and the error is probably coming from the SQL Server, but "why" is related to the code that you execute (the third party SP)