simulate this circuit – Schematic created using CircuitLab
Figure 1. The likely cause of the low voltage reason on the blue phase.
It sounds as though your supply is from a star (European term) or wye (American term) connected transformer. Normally the star point, marked 'N' for neutral, is connected to earth to "neutralise" it.
In your case it seems as though either
- the blue phase has been earthed for some reason or
- that the transformer secondary star / wye point was not grounded but has now developed an earth fault on one phase.
The second arrangement can be used in the right circumstances to give single-fault tolerance of a ground fault. The circuit will continue to operate (as you have noticed) until a second ground fault occurs. When this is done it would be normal to add ground fault detection to warn of the event. Three bulbs in place of the voltmeters would suffice: when a ground fault occurs that lamp would go out.
The problem with this arrangement is that devices rated for 230 V to ground now have 400 V to ground. This may stress the insulation, causing early failure, and also provides a higher shock hazard to maintenance personnel.
simulate this circuit
Figure 2. Earthing of star / wye point on transformer. This is standard European practice.
I'd recommend understanding the design intent, examining the system and repair, if required.