Overview of the issue
If you are using Jupiter X theme, in some cases the customizer may take a long time to load or may not work at all due to a timeout error. This happens because the theme mods option on the database is too big.
The issue has been reported to Jupiter X theme developers. As a workaround, you can follow these steps to be able to load the customizer properly:
1. Go to Jupiter X → Control Panel → System Status
2. Scroll down to Theme Mods Size
3. Click on the Clear button (it may take some time to finish)