This is my problem: I can always open one PDF document in Acrobat Reader from Explorer, and sometimes more (especially soon after Windows restart). However, after some time it starts to fail: when I double-click a PDF file, there is no immediate reaction, and some time later I get an error dialog saying
Fatal error: Acrobat failed to connect to DDE server
I can open files without problem: 1) from Explorer after closing all other instances of Acrobat Reader; 2) from Acrobat Reader's
File > Open menu.
Googling shows many forum threads about this error. However, most are about problems working over network; this is on my personal computer. Disabling "Enable protected mode at startup" works for some people, but not in my case.