Tabular report error: java.lang.NullPointerException AND can

A number of our tabular reports are displaying system error: java.lang.NullPointerException when trying to view it. If I go in and Edit the report and Save it, the error goes away - so that's not too big of a deal. This only was displaying the error for a few of the tabular reports (not all of them). However, another thing I noticed (that might be related to the first issue) was that some of our tabular reports used to be grouped by a "Group of Checkbox" field. However, since Monday - the grouping was removed and the option to group by the "Group of Checkbox" field is no longer available. Are we no longer able to group by Group of Checkbox fields for reports/views? Thanks, Mike