Ok. One step back.
I can see the stripslashes error if I use Advanced search on an element in a repeatgroup (Advanced search is spinning in this case, the error can be seen in the network response).
I didn't test but I assume this is also the case with php7. Advanced search has issues (e.g. jdate not working, filter type auto-complete not working etc.)
I can't replicate the error in the normal filter even if additionally Advanced search is enabled.
But if it's working for you with Advanced search disabled it's ok.
Update-col with user selection is using the filter layout of elements which are set to be list filters. Which often doesn't make sense (e.g. date element with range filter is showing as range; field with dropdown filter as a dropdown so you never can set a "new" value etc).
So what is your exact setup (which elements are included in the update-col, which elements of these are also list filters (and which filter type), which element did you try to update etc.)?
"error 500": What do you get in your browser console in this case?
Did you clear all J! and browser cache?
It it really working with exactly the same setup with php7 (i.e. just by switching your site's php version)?