Inline Editing is not supported for this cell

I have a report in which I have 2 plain text fields.

The fields are on the page layout and has all the FLS needed.

The user is a system admin and has edit access to the object too.

All of a sudden I'm getting the error 'Inline Editing is not supported for this cell'.

I was able to do inline editing till the past week.

Any pointers on why it might have started behaving weirdly?

Please note the object has more 12 million records and the report has filters applied.

Thanks