Hey there!
So I upgraded my beta version from 1.7.3.x to 1.8.0.486 and tried to open a file - first I got the usual "Open non-Beta-document" dialogue - although this file was created with a beta version of Affinity Designer. After hitting either "Continue" or "Make Copy" I get an error dialogue (see attachements). Other documents which were created previously with the same 1.7.3.x beta version of AD open just fine in 1.8.0.486-b.
Greetings, Joe