Looks rather a PBX problem in this case, so let's check this way:
- what is the 3CX client as in its menu Settings/About?
- what is the PBX version as in the client menu Avatar/ Connection details?
then
- does this chat appear also on WebClient or Android if you connect them on the same account?
- if yes, can you delete them there?
I would need the client verbose logfile:
- open client menu Settings/Troubleshooting and set Verbose logging to ON
- terminate the app by swiping it up
- launch the app again
- go to chats and there try to delete by swipe left
- go back to menu Settings/Troubleshooting and send the log report to your email address then upload it to a dropbox or google drive and send me the link via PM
Thank you!