Cannot disable a SIM card in dual SIM manager

[viennesewaltz]

I have a Samsung S20+ with dual SIM. In the Connections menu there is a feature called SIM Card Manager which enables you to disable one SIM and keep the other one on. But this is not currently working for me. Both SIMs are toggled to 'on', and when I try and toggle one of them to 'off' I receive an error message saying "Unable to change SIM card. Finish sending multimedia messages and try again". I am not sending any multimedia messages. I have force quit the Messages app, have restarted the phone and have reinserted both SIM cards, but the problem remains. Any ideas? Many thanks.