Referring to the initial post, the main problem is created by contact arcing of the power contactors rather than inductive kickback of relay coils. Inductance of e.g. AC motors will increase contact arcing at switch-off, but the problems may be present with large resistive loads as well.
There a basically three ways to reduce contactor related interferences:
- reduce contact arcing, e.g. by snubbers or varistors. It's not common or suitable for large (kW range) contactors.
- separation of logic/processor and power interface circuits. According to your reports, you have optocouplers providing the separation. To be effective, you also need separated grounds and power supplies. Without inspecting your actual setup, I won't guess about effective implementation in your ciruit.
- "hardening" of the logic/processor circuit against interferences. Involves solid ground planes or at least meshed ground nets for the processor board, power supply filtering with surge arrestors, usage of common mode chokes for external cables, input filters for external input signals, protection (z-diodes, varistors) for output signals.