In versions 3.7 and prior, there was no support for multi-value parameters in a Metadata query. If you have a report created in an earlier version, which contains a Metadata query and an "exactly matches" condition, the report will continue to work as is; however, if you try to change the parameter from a drop-down to a multi-selection type, such as a checkbox containing more than one value, the report will fail.
To resolve the problem, simply open the query for editing (Query Editor) and click OK. This adjusts MQL query to use the EQUALS function instead of the = operator. No additional changes are necessary.