Hi Olga,
unfortunately using the low end product of a high end provider, I have to do my own troubleshooting. So my first thought was to disable all non essential plugins, and re-enable one by one and test. This led me to discover that a plugin I had to force https was causing the problem. With it enabled I could no longer customise. I could not recall the original reason for having this, but tested without and all interaction on the site is https, so I have gone ahead and removed it and updated my prod environment.
I have however encountered another issue with the woocommerce admin plugin provided by woocommerce, enabling this and trying to populate the analytics brought my site to it’s knees, so i have removed that, and everything seems to be working as expected!