Your problem is the max files setting in Linux. Currently, trhe kernel does not allow to open any new files and this causes ispconfig to fail. See here on how to increase the number of open files in Linux:
As a sidenote, you posted errors from the interface on the slaves. Slaves may not run a interface. If you would login to the interface on the slave and add or change something, it will corrupt the database. So never enable the interface on a slave server, a interface shall be installed only on the master.