Sadly you closed our thread too quick… so here we go again.
The old start posting of our threads describes the problem:
we got customer reports having problems while using Safari on MacOS. We did a lot of debugging and came to the solution that it’s a problem with trying to use multi-select forms on X theme with Safari on MacOS. There are no problems with other browsers or with Safari on Windows.
To exclude cases of custom css or php we tried with X theme “out of the box”: errors persist. We tried with Twenty Nineteen theme: errors gone. We also tried without most of our plugins.
As an answer to your request I will add login details to our staging for you to take a closer look.