Bit of an odd one. I have a network of around 100 sites, each of them have a mappd domain. On just one of these sites, I am unable to load the customizer - all other sites act perfectly fine. I have narrowd this down to an issue with the domain as if I were to remove the domain, the customizer loads properly.
I took a look at my debug file today and saw this error:
[12-Jan-2017 02:19:52 UTC] PHP Fatal error: Maximum execution time of 30 seconds exceeded in .../wp-content/plugins/domain-mapping/classes/Domainmap/Module/Mapping.php on line 459
It's a very strange issue to just have on one site, but can you perhaps suggest why it may be happening?