Aura HD Sideload using web browser broken for larger files

Hi,
as my kobo aura hd just updated to 4.3.8842 and I noticed that the sideload of epubs using the web browser for epubs larger than ~1.4-2.0mb stopped working. It still works fine for files up to ~1.2mb (largest epub i have which still works, everything else is already like 1.7mb which do not work).
It shows the downloading popup with the progress bar but nothing happens, after abort it creates a 0 byte file.

I basically have a local file server running on my nas with file listing enabled and copy my epubs there and use the webbrowser to download them. I tried another webserver on my desktop machine but it didn't help.

Factory Reset didn't help.

I downgraded to 3.19 and it works again.

I looked for this bug on the appropriate firmware threads but couldn't find anything.