Snapshot backup throws an error

When I try to run the backup, I get the following error:

failed to open stream: Permission denied in /var/www/vhosts/****.ch/httpdocs/wp-content/plugins/snapshot/lib/Snapshot/Helper/Session.php on line 71

Warning: file_put_contents(/5152505846436c55465f5f_session.php): failed to open stream: Permission denied in /var/www/vhosts/****.ch/httpdocs/wp-content/plugins/snapshot/lib/Snapshot/Helper/Session.php on line 71

In the screen of the Snapshot Managed Backups, I get "-1" error.

When I browse the pages in the plugin, I get:
""ERROR: Cannot create snapshot folder. Check that the parent folder is writable /home/meinsola/public_html/wp-content/uploads/snapshots"".
All the 3 folders wp-content,uploads & snapshots have 755 permissions.

I am right now unable to backup my site. Please help me with this.