Solve a MySQL problem on my webserver

I run a small Virtual Web Server under Debian Wheezy and Virtualmin/Webmin. Since yesterday it has a problem with the MySQL server. The Virtualmin System Information shows that the MySQL server is running, but I'm always getting errors like those: "Manual SQL Error: [1016] Can't open file: './iwpadminpanel/[login to view URL]' (errno: 24)" "Manual SQL Error: [23] Out of resources when opening file '/tmp/#[login to view URL]' (Errcode: 24)" "SQL desc `user` failed : Out of resources when opening file '/tmp/#[login to view URL]' (Errcode: 24)" etc. Server data: – Virtual Server under Virtuozzo – Debian Wheezy 64 bit – MySQL version 5.5.35 – Administration: Virtualmin/Webmin