Thank you so much for your support.
I've reduced the flow to a SSCCE (see below).
Note that the query is not the one from my previous post, but the error msg remains the same.
[{"id":"4547bc84.5880d4","type":"tab","label":"Forum","disabled":false,"info":""},{"id":"6b0f29f2.9044d8","type":"inject","z":"4547bc84.5880d4","name":"Jede Stunde","topic":"","payload":"","payloadType":"date","repeat":"3600","crontab":"","once":true,"onceDelay":"0.5","x":140,"y":60,"wires":[["145f7910.9b8617"]]},{"id":"145f7910.9b8617","type":"function","z":"4547bc84.5880d4","name":"QryDeleteData","func":"var out = \"DELETE FROM [mqtt_datablockManagementLog] \"\nout = out + \" WHERE DATEDIFF(DAY, zeitstempel, getDate()) > 7;\"\nmsg.payload=out;\n\nreturn msg;","outputs":1,"noerr":0,"x":420,"y":60,"wires":[["b4771efb.8dcc4"]]},{"id":"b4771efb.8dcc4","type":"MSSQL","z":"4547bc84.5880d4","mssqlCN":"9deeb56.dd4c3c8","name":"BereinigeLog","query":"","outField":"payload","returnType":"0","throwErrors":"0","x":650,"y":60,"wires":[["866efd5e.07b1d8"]]},{"id":"866efd5e.07b1d8","type":"debug","z":"4547bc84.5880d4","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","x":880,"y":60,"wires":[]},{"id":"9deeb56.dd4c3c8","type":"MSSQL-CN","z":"","tdsVersion":"7_3_A","name":"DataWarehouse","server":"172.23.4.55\\GalileoAPS","port":"1433","encyption":false,"database":"DataWarehouse","useUTC":false,"connectTimeout":"","requestTimeout":"","cancelTimeout":"","pool":"","parseJSON":false}]