How are you issuing the processing command? Are you using a SQL Server job with Type: SQL Services Analysis Services Command?
I dimly recall that error but I haven't seen it lately. I have had better success using TSQL against a linked server.
DECLARE @XMLA NVARCHAR(MAX) = '{"refresh": {"type": "full","objects": [{"database": "MyDatabase"}]}}'
EXEC (@XMLA) AT [MyLinkedServer];