When a volume is spread across several members of an Equallogic group, the check_equallogic plugin has a problem identifying the correct volume (as it is seen several times).
The newest version 20131122 fixes this bug.
Furthermore it seems that the firmware version 7.0 for Equallogic breaks the volume checks (vol and volumes). The SNMP OID which used to return the actual usage of volumes now returns a plain "1" as integer. Of course that breaks the calculation of usage. To be verified and followed...
Big thanks go to Cyril Epiney for informing me about the issues and the debugging.
Update November 25th 2013:
As it turns out, there was no issue with firmware 7.0. False alert!