Ive gotten the tip to send an error report via the Samsung Members app installed on your phone (or re-install)
Do it like this: boot up the phone, and enter the home screen. Then just wait for the Sim card manager to come up, and click ok/ready. Then go directly to the Samsung Members app, and select bottom option Support, then Send Feedback, then select the option Error reports, and enter shortly the mentioned popup of the simcard manager, and send the report. (Im doing this from Dutch, don't 100% know what the english option names are, but this should be enough to find the options)
This way, Samsung is getting the reports of the same issue in an structured manner, and as long as we all send the reports, this would pick up on their radar.
OK, here is a solution (more workaround, but is works and that's important):
The issue is probably caused by 1 SIM card in slot 1 and SD card in slot 2 (probably also without SD).
Try this: remove the SD card and put any SIM into slot 2 so you have two SIM cards active. Confirm the dialog and restart the phone. The dialog can but not have to appear. Remove the SIM2 and return SD card back.
Now the dialog shoudn't display after reboot (works for me)
Every time I switch on my phone the sim manager pops up, no apparent settings to change, I click done and back to normal. But why and how to stop it? Or maybe there's a setting I need to to change.
Everything works fine
If this is still happening, you could try a force restart. This sometimes helps with niggling issues and worth a try before either sending the phone back or factory resetting the phone.
How to force restart.............
"Press and hold the Power and Volume down buttons at the same time for at least seven seconds. Your Galaxy S20 will now restart"
I have the same issue on my ultra. Tried a force reboot but no change, may have to bite the bullet and do a full reset if that's the only option. Anyone else resolved this issue ?
I won't be resetting as the issue is not a problem just a glitch, resetting is quite a pain and not guaranteed to fix.