We are in the process of migrating our organisation from SP2013 to SP2019. We have 3 environments setup, with 2 Web Applications in each. DEV is a 2 server farm, UAT and PROD are both 8 server farms. Since the August CU/Language Pack we have been unable to use any of the existing modern site collections due to JS errors. The errors cause certain links in the site settings menu to not work, and the Edit page buttons are not rendered, basically mean users have no means for editing pages.
What I have been able to determine is that if I create a brand new web application and create an OOB Communications site, there are now issues. It only appears to affect modern site collection in the existing Web Applications.
There are no customisations present in these farms. No web parts or app customisers. They are basically new unmodified farms, so we are at a loss as to what may be the cause. As I mentioned the sites were perfectly fine up until the August CU, where we noticed after patching the severity of the issues.
I have even tried deleting completely the existing web applications (in DEV) and recreating them with the same names, however this still results in the same console errors and unusable Modern sites.
I've posted this issue in stack exchange without any success, would love any insights into what the issue could be.