Cannot use front-end builder - will not save

Im tryng to edit a header using visual builder - ultimate header plugin, have tried over and over again, have followed all Divi insturctions i.e. making sure I have enough memory, php, etc. - all good. tried de-activating all plugins. Also tested visual builder on a pageand that works just fine. So the visual builder works everyhwere else, just not in the Divi Ultimate Header or Footer plugin. Here's the error message - i will also attach a screenshot: (new installation, only 4 plugins and the only one active right now is yours.

An error has occurred while saving your page. Various problems can cause a save to fail, such as a lack of server resources, firewall blockages, plugin conflicts or server misconfiguration. You can try saving again by clicking Try Again, or you can download a backup of your unsaved page by clicking Download Backup. Backups can be restored using the portability system while next editing your page.

Contacting your host and asking them to increase the following PHP variables may help: memory_limit, max_execution_time, upload_max_filesize, post_max_size, max_input_time, max_input_vars. In addition, auditing your firewall error log (such as ModSecurity) may reveal false positives that are preventing saves from completing.

Lastly, it is recommended that you temporarily disable all WordPress plugins and browser extensions and try to save again to determine if something is causing a conflict.