I have determined that the problem is that the memory allocated in the new version of the compiler is seemingly but a fraction of what it was in the previous one.
My app compiled when I disables all the logic (almost 6000 blocks). Through trial and error, I established that my app would compile with 1934 logic blocks, but fail if I re-enabled a mere 9 block logic.
Also, I established that the compilation would fail if I had those 1934 logic blocks but had added a single button (without any supporting event handling), showing that memory is shared between components in the layout and logic in the block page.
==> if your app failed to compile when all the logic was removed, it probably has more than the 217 components (arrangements, buttons, labels, canvas, whatever) mine has in Screen1 (for the record, the compiler limitation applies to individual screens, in isolation. When I tested with 1943 logic block in Screen1, disabling 100 blocks in the much smaller Screen2 had zero effect and compilation still failed; Screen1 size was the limiting factor) OR your components are heavier than mine (I assume a Webviewer to have more attributes and therefore bigger memory requirement than a label has).
I feel that the diagnostic is there. The solution is 100% on the side of Thunkable.