Posting this here because I'm sure this is connected to a recent update.
If you post a link now, it might end up looking like this...
#mce_temp_url#
...and if you selected some text specifically to indicate what the link was about, then that's a bit annoying.
If you want to correct it then here's how...
Select your text or image for your link and click on the insert/edit link button on the toolbar, when the insert/edit link dialogue box opens, your selection will become unselected.
If you click away from the insert/edit link dialogue box, do not close it!, then re-select your text or picture, you can then go back to the insert/edit link dialogue box, which might not be visible if you have your browser maximised, so either go down to your taskbar and hover over your browser icon to see other open browser windows to select the dialogue box, or use the task switcher shortcut, ALT + TAB, to cycle through the other open windows. Once back in the insert/edit link dialogue box you can fill in the link URL and hit the insert button to complete the procedure.
Many, many thanks to BPeterson8256 for pointing this out to me. I had completely overlooked the fact that any selection made, needed to remain selected while filling out the insert/edit link dialogue on this step of making a link.