>I don't know if there is anything special configured on those switches. But I will pass your message to the network engineer and let him decide what (if anything) could and should be done on the switch.
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>I thought about the cables too. But since loading application from two PCs connected via different switches does not present the problem (and they both probably still use the same cable connected to the server), doesn't it rule out the issue with cable?
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>Antivirus is not the issue here because the problem is only when 2 people connecting and using the same switch.

I understand that you came to the conclusion that network problems arise only when a single switch is involved; but with normal functioning of the switch, a user on one switch port should not interfere with a user on another switch port. It therefore ocurred to me that the conclusion reached might be premature, and should be taken with care.

A high-capacity switch should not become "overloaded" if two users connect to it at the same time; a bandwidth bottleneck on an individual cable is the only issue I can think of.

But let me see whether I can think of other issues.

You might try connecting users' network cables to different switch ports - perhaps one of the ports is damaged, configured for a low speed, or with some other special configuration command.

The switch itself might be seriously damaged. Not very likely, I believe (without the switch stopping completely); I estimate the possibility of individual ports being damaged far more likely.

And, of course, I might be missing some other network technology which I know about, or which I don't know about; that's why I suggested checking the configuration - or resetting to factory default (only if nothing special has been configured yet).