Took me a while to get back to this, yes, your pointer was spot on, thanks for providing the link.
I am a little further now - I had to convert all my tables to InnoDB (yes some were using MYISAM!). The current error seems harder to overcome for me:
Updating database schema (2020020101)... [FAILED]
ERROR: Error in DDL upgrade 2020020101: [3886] Could not change column 'mailbox' of table 'cache_index'. The resulting size of index 'PRIMARY' would exceed the max key length of 1000 bytes.
Any chance you can advise?