Lost ability to add block following upgrade to EPiServer.CMS 11.11.3 - Anyone else?

Hi All,

so as the title says, I've updated EPiServer.CMS to 11.11.3 (and as part of that .UI & .Core to version 11.19.0) and when I try to add a new block I get a blank page:

nothing showing in the console in terms of errors. Anyone else?

Alex