Recently, I tried to start a bounty for two of my questions. But the error message "Unable to load popup -please try again" appears every time I start the bounty. I don't know whether it is a error or a restriction on my action. Please have a check on it. Thanks!

Trying to start a bounty, on any question, results in this: Please fix, in the dev tools I noticed a request to non existent URL. This happens in all SE sites as far as I could see.

Thanks for reporting this. It has been fixed across the network now and you should be able to add bounties until your heart is content. Thanks to balpha who smashed the bug very quickly.