Just installed 4.2.1 on Confluence 3.4.3 – results were not good (sad)

Right now, I have lost all my theme work because when page loads, I find the following errors appear in the console:

GET https://devwiki.domain.tld/plugins/builder/layout-resources.action?layoutId=WIKI21&resource=javascript&hash=1306938075329 404 (Not Found)
layout-resources.action GET https://devwiki.domain.tld/plugins/builder/layout-resources.action?layoutId=WIKI21&key=MAIN&resource=combinedcss&hash=1306938075329s2033747_ 404 (Not Found)
layout-resources.action GET https://devwiki.domain.tld/plugins/builder/layout-resources.action?layoutId=WIKI21&key=MAIN&resource=importcss&hash=1306938075329s2033747_ 404 (Not Found)
Home:146 Uncaught ReferenceError: BuilderEvents is not defined

Since I didn't do ANYTHING other than install the upgraded plugin, I am (as I hope you can imagine) frustrated.

Is there any help out there?