Time ago I've deleted a single audio file in Wave Files folder. Now when I open the project that once contained this file, everytime appears a warning that says:
File e:\multisamples\02 - drums\pro cymbals room\paiste signature full crash 16\ at4041 - hard - 001 flac not found or couldn't be loaded.
What to do? I don't want to see that warning anymore.

This is a sample which ships with DimPro.  The path is different from the default location. Is it possible this is a warning message being issued by DimPro? If so, there is a fix but it depends on a few things. A little more info about the project may help.
Start with, is DimPro in the project?
If so:
Does the path look reasonable for the relocated multisamples
Is there a drum program loaded? which one?
 
 

Yes Scook, there is Dim Pro playing a piano part. EZ Drummer is the main drum instrument.
I think that time ago there was some drums played by Dim Pro and maybe deleted.

I am guessing there there is still an instance of DimPro in the synth rack trying to load that sample. If the synth is not in use delete it from the rack.

Check to see that the instance of Dim Pro in question is really gone. I constantly deal with vsti's that don't have a track but still reside in the synth rack. Select it and click the big red X in the Browser button bar.
 
Especially frustrating is opening a 32 bit project in 64 bit Sonar. Dim LE will load but not function regardless of whether it's the DXi or VSTi version. So I am forever replacing these when I open old unfinished 32 bit projects in 64 bit Sonar. UGH! A nice feature is right-clicking to replace the synth. Not so nice is you still have to find and reload the samples into it.

Hmm! He who writes shorter sentences gets to post sooner.

i think it's good practice to NOT delete files from the audio folder of a project :-)

Assuming that E:\multisamples is still a valid path, re-installing Dimension Pro should solve your problem. 

I agree, don't delete stuff, we don't use 20 gig hard drives anymore. 

Hi all there,
scook and soens suggestions were definitively right. I've deleted  two previous DimPro instances in the browser bar, saved the project and  then reopened and ...zac!!!
 The nasty message has gone. Thank you guys for your help.