Question:

My existing report about ticket time log & billing has recently stopped working with an error about "Unknown column reference tickets.charges.comment".

Answer:

The variable to include a ticket report comment changed in Deskpro #413 when it became possible to add custom fields to charges.

Update your report, replacing tickets.charges.comment with tickets.custom_data[1].

If that doesn't work, check the ID of the comment field under Tickets > Time Log & Billing > Fields.

Replace the 1 in tickets.custom_data[1] with the number shown next to Comment.