Termstore error: "Value does not fall within the expected range" (SP Online)

[Finnair]

Strange error appears in SharePoint Online's Termstore.

Termset has worked fine many years but now it announces an error when marking a document with a term from Termstore: Value does not fall within the expected range > the term won't be saved within the document. This applies to all the terms. And there is no "range" within the terms.

Nothing has been developed so the Termstore and Libraries are OOTB.

The site wher this problem appears was migrated to another tenant. But this problem appeared 6 months after the migration.

[#2]

I'm experiencing the exact same issue, except it has occurred immediately after migration. The termsets were migrated using ShareGate.
Note - if I create a new managed metadata column in the affected list, assign the same termset to this new column, then select a term in this new column, this error does not occur.