Email Sending Failed - Invalid user id provided (Subject: [Ticket ID: {$ticket_id}] {$ticket_subject}) - errors in activity log for some ticket responses

[webio]

Hello,

does someone is experiencing:

Email Sending Failed - Invalid user id provided (Subject: [Ticket ID: {$ticket_id}] {$ticket_subject})

error in activity log for some of tickets? Interesting thing here is that I don't have any email template with title "Subject: [Ticket ID: {$ticket_id}] {$ticket_subject}". They are all localized without using any additional languages. So far I didn't found a key for getting this error in controlled scenario. It just appears for some tickets and prevents of sending them to client (while marking them as answered in WHMCS and not showing any errors during ticket responding/submitting response). This has started to happen after upgrade to 8.x.

Thanks

[webio]

Ok so I've done some checking and this error occurs ONLY for opening ticket using wwwform from user which is not a WHMCS client/user. This kind of ticket has in table in userid column value NULL instead of "0".

[xyzulu]

See