My name Istvan fro Hungary and I met with this problem in my practice. If the protection are designed well . look aroud the installation. The output of the MGE ups (tree phase ) mostly has a neutral in the output , and there is an earthing . One case a costumer led the tree phase and the earthing from the UPS to his devices , and the star connetted output transformer's star point were not conneted. There arised a virtual star point , the earthing wire , and the different phase voltage set by the ratio of the the loads by phases . The line voltage were constant, only the phase voltage were changing between 100 and 500 V . The result was same what You described.