Weired behaviour - clicking files does nothing

[Lockheed]

Every now and then this happens:

If I click an icon, say an mp3 file (same for every filetype: avi, doc, txt, etc), on the desktop or inside pcmanfm file manager, a new tab appears on my taskbar, for example "VLC media player". It stays there for 10 seconds, and then disappears. Nothing happens.
The same goes for docs, pdfs, program executables, etc.

Once it starts happenning, I need to restart my computer. Otherwise I won't be able to run anything UNLESS I use Start Menu to open a program and then use "Open file" option within that program.

What is this? I tried killing and restarting pcmanfm, which also manages my desktop, but it does not help.

[Lockheed]

Now I now the cause of it. Every now and then PCmanFM crashes for unknow reason. Once it does, I need to start it again as my desktop and file manager, but clicking stuff no longer works and I need to reboot.

Any solutions come to your mind?