error after moving to VPS : local / cache-vignettes / /. Ok.

Hello
I moved SPIP and data to a VPS. Everything went smoothly. However, when i try to log onto the private area I get an error :

“verify write permissions

The system Encountered an error When Trying to write the file local / <code>cache-vignettes / /. Ok. As a site administrator, please verify write permissions of the directory local / cache-vignettes.”

I searched for solutions and tried a few, even contacted the VPS provider. So far no luck.

I use SPIP 2.10 + Ahutsic. The VPS uses Plesk 10.3.1.

Many thanks