I investigated further:
2020-09-02 10:55:53: Caught an exception: --> Smarty: Unable to load template tygh 'common/letter_footer.tpl' in 'upgrade/backup_info.tpl' <--
2020-09-02 10:55:53: PHP Fatal Error: Uncaught --> Smarty: Unable to load template tygh 'views/upgrade_center/components/notices.tpl' <--
thrown in /var/www/vhosts/****.com/shop.***.com/app/lib/vendor/smarty/smarty/libs/sysplugins/smarty_internal_templatebase.php on line 129
It sounds like you have a corrupted installation from your description. Not sure why a 'letter_footer.tpl' would be in the 'responsive/templates' directory tree rather than 'responsive/mail/templates'.
There should be more to your smarty error message.
nightmare... I'm getting a Access denied: Possible CSRF attack now..
I'll just install the lates version and be done with this..
Update on CS cart has always been a pain .. back in the days I was doing had to do them manualy ...
the the auto came in with every time lots of 'fix to be done"..
I let it slide for 5 years as I could not ge bother to have to redo restart from scratch again ... seems I'll have to any how :(
I solve the issues with the latest install ! (may be it will serve other)
during the installation I chose to install optional import plugin ( I thought it could be handy to import previous version of the shop)
this was a mistake... and this plugin gives (me) a 403 error..
I disabled It and the "depreciated" import plugin worked fine.