Expressions that don't work...

[dbDavideBoscolo]

This doesn't work...

[Moistoff]

Try to click on 'Layer Name', it will switch naming on 'Source Name', and look if there is another name of the layer, then typed this 'Source Name' in your expression

If it doesnt work, try to rename your layer, I have deal with the same problem and renaming was helpful

[Moistoff]

Using pickwheep its more useful than writing simply expression by yourself, cause there`s cant be errors 

[Mylenium]

Check, whether there isn't an extraneaous space at the end of the "MAIN" layer name or something of the sort.

Mylenium

[dbDavideBoscolo · Accepted solution]

I checked the name, tried a new one, double-checked that the name was unique, used the pickwheep and restarted After Effects before posting. Nothing worked. 
This morning I updated AE and the problem doesn't exist anymore.