Initial tests fails on startup most of the time. The hotter /faster the startup is, the more likely it is to fail.
Error is very frequent but not constant, and occurs during download phase and behaves like a deadlock - no more downloads are resumed, and initialization fails.

By walking up the stack I have found some kind of parallel download mechanism.
A constant 'maxParallelDownloads' is defined here: https://github.com/dotnet/runtime/blob/release/9.0-rc1/src/mono/browser/runtime/loader/globals.ts#L94

initial tests show that if I change this value to 1 (loaderHelpers.maxParallelDownloads) during the load phase, things seems to work out fine, and the "deadlocky" situation does not appear.

Some additional testing is needed to find a good value for this constant, or see how the deadlocky situation can be avoided in the first place. may involve rewriting my bootloader.