In post editor, if you click on 'insert link' button, then this dialog doesn't allow some kind of links. Unfortunately I can't say, what it checks and can't give good examples - but I had yesterday problem with one long link and this address above www.photonstophotos.net/Charts/PDR.htm#Sony%20ILCE-7M4 says 'incorrect' either (I can add https there or change %20 to space - no difference).
I think the entry fails at the # symbol. Clip after .htm and it’s accepted
Is there a reason the entry box is needed beyond making it easy to add a title. Cut and paste into the message text works fine (in contrast to DPR behavior)
What I currently do is use link button but remove from pound sign on and then paste that in after the link is made.
A bit tedious pasting twice and removing excess isn't too bad.