Have you tried reinstalling OpenMPT so that file associations are reset to their default state?
This is a Windows error message, not from OpenMPT, and there are appearently various reasons for it (just search for the error string on your favourite search engine). Maybe this helps? http://answers.microsoft.com/en-us/windows/forum/windows_7-windows_programs/there-was-a-problem-sending-the-command-to-the/56ee1743-b476-43a5-9b00-169b8c6ef37a

Sorry, didn't intend to never respond. I tried the reinstall and the various suggestions on that Microsoft page to no avail. I'm probably not going to bother pursuing it any further--I just figured I'd throw it out there in case it was something weird in MPT. I'll just ignore it as I always have.

I've just tested this bug on my end, and everything works as it should.

Now, I have a fair bit of understanding how file association works in Windows 7, and have had quite a few occassions when a file could not be opened anymore by its associating program. It is not that hard to fix this, but it requires editing the registry.

Please note, editing the registry can mess things up badly, so only continue if you feel comfortable enough with computers.

Open regedit by pressing the start orb, and type in regedit in the search field.

If you feel insecure and want to make a backup first, we'll describe that step here:

• Open the Computer tree.
• Right click the first node (HKEY_CLASSES_ROOT) and select export.
  We're not going to mess with the other subkeys, so we don't need to backup them. The entire registry is pretty big so that would just create a large backup and take a while to complete anyway.

Now that the backup is done, lets perform the action.

• Open the subkey HKEY_CLASSES_ROOT
  The first item is a . This key applies to all files, and is used to add extra options. You only need to concern yourself with this key if you'd right-click the file and choose another item that is not the first (open). Given that you are double-clicking, we can leave the alone.
• Find the extension that causes problems (in your case .it)
• Click the key.
• On the right it will say (Default) with type REG_SZ. Read what the value is in the Data column. If this is (value not set) you can skip the next steps until it says #here#.
• search for the value found above in the same place you found .it key. You can type in the first letters of this and you probably jump there.
  Rightclick that key and choose export. Save the key as backup in case you messed up.
• Rightclick again and now delete the key.
• Go back up to your .it extension (you can type .it quickly to get there too)

If you had (value not set), you can continue from #here#.

• Rightclick the .it key and choose export (backup in case you messed up.
• Rightclick the .it key and delete it.

At this point, the .it file is no longer ascociated to anything, but windows may still be confused.

• Reboot your computer.

• After windows boots up, open an explorer window, find an .it file and double click the file

• Windows will now say that the file is not ascociated and asks you with what program it needs to be open.

• Select the 2nd option: Select a program from a list of installed programs.

• If the Recommended Programs nor the Other programs include OpenMPT / ModPlug Tracker, press the browse button below it, and navigate to where you have mptrack.exe

• Select OpenMPT from the list, make sure the checkbox for Aways use the selected program to open this kind of file is checked, and press okay.

Now your problem is fixed.

If anything went wrong during these steps, you can doubleclick one of the made .reg files to restore those values.

I suspect your problem comes from missing quotes in the file ascosiation, which makes any file with a space in it being treated as separate arguments, thus failing to load the song.

Reinstalling OpenMPT would do exactly what you have just described, except that the essential key is "OpenMPTFile" - the other keys, such as .it, .xm, ... just link to it.

Interestingly, I can reproduce this when OpenMPT has to display a keybinding error message on startup. It doesn't happen with "plugin not found" error messages, though, probably because those appear way later.
I'm not entirely sure what the problem is, especially since I suppose that OpenMPT doesn't generate any warning messages on each and every launch on your end. Just a wild guess, does it maybe help to disable the splash screen (http://wiki.openmpt.org/Manual:_Hidden_Settings)?

I have not tried any suggestions, but I want to point out that the module still loads after I tell the box to go away. It's really quite strange.

Ok, checked through the registry. Everything is set up fine in there (I'm familiar with file association structure as well). If I may ask, what does this extra "ddeexec" key do? It's the only thing different I see from a manually created extension (and thus I'm inclined to skip the manual recreation as it would literally make an identical entry save for some names and this key).

Another clarification: it happens EVERY TIME I open a module through double-click, on ALL COMPUTERS I've ever had MPT installed on. It NEVER displays this error when loading into an already open MPT window, and the error does NOT prevent MPT from loading the module--it just slows it down.

Please try my suggestion (hiding the splash screen) instead. As I said before, LPChip's suggestions are irrelevant, as this also happens with a perfectly normal registry setting here as well. The problem seems to be that Windows wants to pass the file via DDE (which the "ddeexec" key is for), but in some cases it thinks that the DDE process failed.

<blockquote> It NEVER displays this error when loading into an already open MPT window, and the error does NOT prevent MPT from loading the module--it just slows it down. </blockquote>
Yes, I am aware of that, and it all boils down to a problem with DDE. DDE is actually required for the whole "open in existing instance instead of opening a new instance" thing, and when the tracker is already open, DDE seems to work in all cases.

So, does disabling the splash screen help or not?

http://blogs.msdn.com/b/oldnewthing/archive/2014/06/20/10535855.aspx
This seems to be related. And my previous question goes into the same direction, so it would be useful if you could answer it.

No response - assume fixed.