An unexpected error occurred during the export

Good afternoon.
There is a local server. Everything worked like clockwork, but when it was necessary to export the data, an error appeared on any attempt to export the data - " * * destination: the Export file will be available for download. An unexpected error occurred during the export. We apologize for the inconvenience. Please contact our support team at**."
Version 19.8 and PostgreSQL 10 were installed. Trying to solve the problem updated to the latest version From unpleasant events since installation V. 19.8-there was a long power outage and the UPS did not protect.
Tell me what may be the cause of the error?

From your description you were performing an update from v19.8 to v19.12.6.
During this process the database is migrated to a new format, which may take significant time, depending on your server performance and the amount of data.

It follows further that there was a power outage sometime during the operation, which may have left the database in the corrupt state. A corrupt database may be not suitable for exporting the data, and for migration during the update to the new version. Since this is a local server, we have no way of telling more.

Try to recover your installation from the backup to restore the before-update state (or before the power failure state, whichever gives you the data at export - this would be a good sign of database integrity).