Check in the log files to see what might be causing the error with Teams - C:\ProgramData\Veeam

In here there will be another folder for O365 and then a job folder with logs to check.  Hopefully one of those helps.

Hi,

Are you sure that teams site has still an owner mailbox?

Hi! The group mailbox corresponding to a Team requires an owner to have its mailbox backed up, otherwise the backup warning will appear. Please assign an owner to the group mailbox and try the backup again.

Thanks all, I've checked the mailbox associated with the team and I'm an owner. Just to double check though that I'm checking this by going onto the team mailbox in Office 365 admin and checking the “owners” group (myself and another Office 365 admin are in there) This is the part of the log I've found (I've replaced the team name with testteam): 

�16.01.2023 00:20:26]   52 (22788) Loading applications of team: testteam (team ID: add2d59f-2fa1-4f2d-9793-9f2ca237666b)...
)16.01.2023 00:20:26]   52 (22788) Team testteam processing finished (team ID: add2d59f-2fa1-4f2d-9793-9f2ca237666b, users loaded: False, channels loaded: False)
a16.01.2023 00:20:26]   52 (22788) Items processed: 0. Items total size: 0
i16.01.2023 00:20:26]   52 (22788) Processing finished with warning: Team site not found: testteam

I would suggest opening a ticket with Support for further troubleshooting as that information is not enough to go on and they can provide further assistance.

Indeed, the Team owners and members are selected through the admin center - https://learn.microsoft.com/en-us/microsoftteams/assign-roles-permissions. You can collect the logs, as described here - https://helpcenter.veeam.com/docs/vbo365/guide/vbo_exporting_logs.html?ver=60 and send me them directly to nikita.kozlenko@veeam.com and I will do my best to check them out, but opening a support case is a great idea as well!