The issue exists in all Adobe Reader versions released in 2020.

To establish DDE communication with Adobe Reader, a client needs to know DDE service name. It can be read from the following entry in Windows registry:
Computer\HKEYLOCALMACHINE\SOFTWARE\Classes\acrobat\shell\open\ddeexec\application
This entry is created/updated during Adobe Reader installation.
Adobe Reader 2020.006.20034 saves the following value to this entry: "AcroViewR19". Our application reads this value and tries to establish DDE connections with Adobe Reader. However, DDE connection fails when "AcroViewR19" is used as service name. When the registry value is corrected to “AcroViewR20” – DDE connection successfully establishes. Any update of Adobe reader resets this value back to “AcroViewR19” AND WE BACK TO THE PROBLEM. Note that Adobe Reader DC no longer allows to disable updates.
Simple fix on Adobe Reader side is to save correct DDE service name in the registry, i.e. the name that matches the actual DDE service name.