Installing gp-premium kills Live preview/Customization

Yes,

- all plugins deactivated but GP premium active --> problem still exists
- all plugins active but GP premium deactivated --> all works ok
- choose other theme --> customising is possble
- switched on WP_DEBUG --> no errors shown
- switch on WP_LOG --> no errors shown

Only in the case that both GP and GP Premium are active there is no live preview.

What if you deactivate all of the GP Premium in Appearance > GeneratePress?

What WordPress version are you using?

Sorry it took a while. I am only at this server (intranet) wednesday, thursday amd fridays.

With all plugins active --> No customize
With only GP-premium deactivated --> Customization is possible
Switching GP back on --> No customise

So if you deactivate everything (all plugins + GP Premium) except the GP theme, you still can't access the Customizer?

Any custom functions added to the site? Child theme?

No,

if I deactivate all plugins including GP Premium it is ok, everythings works fine,
if i deactivate all plugins except GP premium customise does not work,
if i activate all plugins including GP theme bit deactivate GP premium it will not work,

so, at the moment I activate GP premium I loose customise.

And what about if you deactivate all of the GPP modules in "Appearance > GeneratePress"?

OK, I have activated all plugins, GP theme and GP Premium bit with all modules deactivated. And this works, I can use customise.

Now, I activated colors, copyright, spacing and typo and it still works. Seems as it will stop after elements is also active

At the same time I see that loading customise is very slow

If it happens when you activate Elements, it means you have an Element added which is breaking the Customizer. What happens if you set all of your Elements as drafts?