You can also copy the link of the challenge and send it to the opponent team admin
How to see missing challenges

Why do i need to do this? Cant chess.com fix this bug?
I suspect if they knew how to fix it, they would have done so by now - I could be wrong though!

Help! I don't find the button "connection" in my settings! Can someone help please?
I suspect that you are clicking on 'settings'. This won't work and the instructions clearly state that you must first be in Live Chess and then (and only then) click on 'settings', which is the gear icon circled in red in the first pic. There is also a red arrow pointing to it. Ask a friend or family member to do it for you.

I've noticed by chance, that my connection was set to "long polling" as I've sent the challenge, which I couldn't see afterwards.
So, the hint to change this setting might help in some cases to accept a challenge, but unfortunately it isn't sure, that this helps the challenger as well.
By the way: Does someone know, why the default setting (let Chess.com decide) might create such problems ...? Does "long polling" really work on all computers ...?
This has been answered, but here are pictures!