I migrated my redmine to a newest version and the newest mysql too. (From 4.1.1 to 4.2.4, and mysql5.7 to mysql 8.0) I did not have any problem with the migration but I got a ticket about the relations does not work, they get 422. I tried too and i got the same error. When you try to a relations you will get 422 and does not matter what relation you try or what ticket try but if you copy the ticket it actually create it.
Thank you for your help.