Windows 10 Fatal Error: Acrobat failed to connect to a DDE server

Today I could not open a pdf document by double-clicking on it in the Windows Explorer. Apparently nothing happened. Eventually, I right-clicked, and selected open with... Chrome. That worked. The default was Adobe Acrobat Reader DC. Later I found, after closing the Explorer widow, three dialog windows underneath. The dialogs said "Fatal Error /  Acrobat Failed to connect to a DDE server."

When double checking while writing this post, I found that double-clicking a pdf document opens Edge -- and that works too. Right clicking, the context menu has "Open" at the top, and further down, "Open with ->" and Adobe Acrobat Reader DC at the top of the submenu.
I just tried the Adobe Acrobat Reader DC selection, and nothing i happening. I minimized all windows one by one to see if a dialog window would appear, but no.

Googling the error message turned up a suggestion to fix a registry enty, but that entry already had the suggested value: HKCR\acrobat\shell\open\ddeexec\application : Default : REG_SZ : AcroViewR10,

What kind of server is Acrobat trying to connect with ? What for? Is Acrobat connecting back to the Explorer window to get the file name?
With no information to go on, how can we troubleshoot this, other than the nuclear option: Reinstall everything?

Thanks