Profile functions not working anymore

After upgrading to SweetDate 3.1 I ran into serious problems and decided to go back to version 2.9.7

Now I got stuck with the weird situation that several profile things don’t work anymore, like editing profile, changing avatar, sending PMs and more. Seems to always go back to page before click.

• I tried without any plugins (except Buddypress of course) to no avail.
• I tried without child theme to no avail. In the child theme I have the new folder Buddypress > activity, groups, members
• When I try with Twenty Sixteen theme, it works!

∆ At the same time my offline version (MAMP) is indeed working well. Same versions of WP 4.9.4 Dutch, BuddyPress 2.9.3, Sweetdate 2.9.7, plugins, php, MySQL.
Only the database content is different as far as I know, with more users, content, etc in problematic online version.

Any idea where to look for the solution?

Thanks a bunch,
Piroo