Hi Martin,
Thanks for reaching out to us! I’d be happy to help you 🙂
It seems like there’s a conflict somewhere. I noticed you replied “yes” to the questions asking if you deactivated all other plugins and switched to the default theme. Did you have the same issue with other plugins deactivated and the default theme? (you may refer to this document about testing for conflicts if you have any doubt).
Also, could you please enable WP_DEBUG (https://codex.wordpress.org/Debugging_in_WordPress) and WP_DEBUG_LOG (https://codex.wordpress.org/Debugging_in_WordPress#WP_DEBUG_LOG) ? This will create a debug.log file inside the /wp-content directory.
After that, try reproducing the issue a few times, so that in case there is an error, it will be saved into the debug.log file. Please share that log file with us so we can see if any problem from there.
Cheers,
Jeremy