Why iDrive's S3 endpoint is being rejected by Discourse?

No. Not at all.

But I’m surprised that today it took the same value, which it wasn’t taking till now (at all times, I was just copying with the ‘copy’ button from idrive site, and pasting here, and today also did the same).

And, now also, I think it sometimes refuses to accept a value, which it was a moment ago.
But all in all, I’m sorted on this point now.