https://bugs.winehq.org/show_bug.cgi?id=50118
--- Comment #4 from mk939@ymail.com --- Thank you for looking into this issue.
Between these two messages I only updated the launcher. Wine version and system configuration are identical. Luckily I found the installer of the old launcher (2020-09-11), and extracted the dll from it. It is indeed a slightly older version (2019-04-09):
https://www.virustotal.com/gui/file/12edffe5be2d9832165f027d7c56a6b046b241ea...
I am really not sure who to blame there, but I'll report back if this issue appears again with the new qt5network.dll file.