Started getting error Data too long for column 'site_pages' at row 1

When trying to add a new entry we are now getting the following error:

SQLSTATE[22001]: String data, right truncated: 1406 Data too long for column ‘site_pages’ at row 1: UPDATE exp_sites SET site_pages = ‘Y….’

This is a multi-site install, the error is occurring on the original base site.

We also have structure installed, new entries are being added as EE entries, but they are not being added to the Structure Listings table.

I’m not sure what information the ‘site_pages’ column actually holds.

Any advice very welcome!