Upstream open circuit: the input current of 6KVA UPS is about: output current /UPS efficiency * 1.25 (charging power) =6000/220/0.9* 1.25=38A, so it is recommended to use a 50A circuit breaker for upstream protection.

The server is restarted repeatedly, and it will be normal without UPS: this shows that the electromagnetic compatibility of this UPS does not meet the relevant standards, resulting in excessive conducted radiation, interference to the connected load and various abnormal working performance. In your case, it will keep restarting. The fundamental solution is to replace UPS with qualified electromagnetic compatibility. Other possible solutions are to install magnetic rings and EMI filters on distribution lines, but the effect is uncertain.