Error "Invalid Value" When Creating Joined Reports with Activities

This error occurs when the 'My Activities' filter is applied to a Tasks and Events block within a Joined Report in Lightning Report Builder. To work around this issue while a fix is pending: First, set the Tasks and Events filter to any option other than 'My Activities.' Save the report. Then reopen the report and change the filter back to 'My Activities.

The error mentioned above is caused by the following Known Issue:

Known Issue: Lightning Report builder - Activities fails to change "Show Me" filter to "My Activities" when part of a Joined Report in preview

Set the filter for the [Tasks and Events] report to something other than "My Activities" , create the Joined Report, and click [Save]. Then, re-open the report and change the "Show Me" filter for the Tasks and Events block back to "My Activities".

Please monitor the Known Issue link above for the latest status on this fix