Time Machine couldn't back up

The Time Machine volume is on an internal SSD, along with user data volumes.

The volume has 82 GB free.

(Of course, I also have a Time Machine volume on an external device.

Time Machine reports "Time Machine couldn't back up ..."

No reason is given. (I regret that I did not record the full message.)

I select Backup now. A backup starts. In Settings, it says "82.18 GB available, Encrypted"

From the progress bar, I'll guess it will be ten minutes or more. I will post again after it completes. (I may need to leave before it completes.; will post again later today.)

I have been getting—each time TM backs up to the internal device—a message saying that TM can not back up one specific file. Thate file opens is two different applications, without error. The file does not change. It is an old file I have not used for years.

TM completes backups to the external device without error. I suppose it does not make a new copy of that unchanged file.