What are your screen settings, in particular bit depth?

32 bit colour depth,
1280 x 1024 resolution,
75 Hz refresh rate.

Edit: Just changed my bit depth from 32 to 16 and back, now everything is fine - so, thanks, I guess. :D

PS: Neat icons by the way. (;

Strange. Can you please run this test version of OpenMPT? It will show you a couple of message boxes at startup (they should all contain the same message). Can you please paste the content of that message box here? (you can hit Ctrl+C while a message box is open to copy its content)
http://sagagames.de/stuff/mptrack_caps.exe

Okay, it showed me eigth message boxes, each with the following content:

Open ModPlug Tracker

BITSPIXEL = 32
PLANES= 1
COLORRES = 24
BLTALIGNMENT = 0
SHADEBLENDCAPS = 3

OK

Edit: Strangely enough, after rebooting, the ugliness came back, and the bit depth change trick also didn't work this time.
Edit: Found a solution after some tweaking, but only for the current session (not as before, where it stayed correct) and it's quite a pain in the ass:
Changing the bit depth, opening OpenMPT in 16 bit mode, then switching back to 32 bits whith OpenMPT open fixes the icons.

Yeah, if OpenMPT detects that Windows uses a bit depth lower than 32-bit (during startup of OpenMPT), it will go back to using pre-multiplied transparency, which explains why it works in that case. However, that doesn't explain at all why alpha-blended icons fail completely for you in 32-bit mode. Have you ever encountered similar issues with other applications?

I don't have high hopes that the small modification I did will fix anything, but can you nevertheless re-download the executable from above and see if the icons look any better?

Not that I remember, really. :|

Edit: You mean the Caps.exe? Re-downloaded, same thing.

Alright, one more thing to try. Re-download the executable from above. It should display some message boxes again this time. The content should be "32 1".

It did:

Open ModPlug Tracker

32 1

OK

But the icons were the same.

Does any of these two executables work for you?
http://sagagames.de/stuff/iconfix.7z

Indeed; even both.

So should I just change the main EXE-File to one of those?

You can do so if you like, I'll incorporate one of the fixes in the official version soon.

Fixed in r4044.

Seems like icons are now broken in RDP sessions with the new approach, so can you please check whether this version here still works for you? http://sagagames.de/stuff/mptrack_bitmap.exe

No, this version is broken again. :/

Anything new?

For the current OpenMPT release, I decided to revert to the old behaviour, because the black, unusable icons over RDP were clearly a greater problem than icons that just look a bit weird, especially since I couldn't find a system to reproduce the latter issue. However, I've tried a new hack that might help you. Does this version maybe work better, or do you get icons with a pink background now? http://sagagames.de/stuff/mptrack_bitmap.exe

Hi there,

neither fixed nor pink - the icons have a black background for me (they are usable however). :/

Okay, next try... can you please re-download the file and see if the background is still black?

No, the background seems to be fixed; however, the icons still look like the alpha channel is not completely correct (nothing heavy, minor artifacts, most notable on the macro and patterns icon).

Alright... I fear that we'll have to leave it at that for now, since I still have no idea why any of this is happening on your system at all, or why the solution that apparently fixed it for you doesn't work in RDP sessions. I hope the lack of alpha channel isn't too much of a problem for you.

No, of course not.
I like OpenMPT as much that I would maybe even use it in command prompt mode. ;)

Anyway, thanks for your effort in helping me.
Keep up the good work!

I've added the last fix that gets rid of the rogue pixels to the official code a while ago. I'll mark this as resolved until I can maybe find an actual explanation and fix for this behaviour.