Firmware upgrade problems: onlykey un-usable (help!)

Onlykey-app version 5.3.6.
Onlykey stick: bought around november 2020

This is an SOS message, as I am currently stuck with a non-functioning onlykey that has worked perfectly for 2 years.

The problem and questions:

Today I tried to upgrade the firmware on my well-functioning onlykey. How long should this process take? After starting the firmware upgrade, the LED was shining white, and the onlykey app stated: “working”. However, it remained in this state for a long time (more 10 or 20 minutes). I had expected such an upgrade to take a few minutes at most. At a certain moment I had to remove it from the USB port because I had to leave.

Problem 1 - no yubikey challenge response: Currently if I attach the onlykey, I can unlock it with its PIN code, and it does type passwords when pressing its keys, but is not recognised as a “hardware device” anymore in keepassxc, (so for yubikey challenge-response.)

Problem 2 - onlykey-app cannot connect to it: If I try to connect my onlykey to the onlykey-app, the onlykey-app states: “Working” and remains in that state indefinitely. So I cannot retry upgrading the firmware. In fact, I cannot change any setting through the onlykey-app anymore.

Onlykey-app version 5.3.6.
Onlykey stick: bought around november 2020

How to fix this problem?