Intermittent SMB/AFP connection issues after switching company network to VLANs

I need a little bit of help to troubleshoot our company network. We have 10+ Macs (different versions) and they are connected to a Mac Server providing a SMB/AFP network share. A few weeks ago, we enhanced our network and split some things up into different VLANs. The network of the Mac Server is now placed in a VLAN, keeping the same subnet. The VLAN is totally managed by switches/access points/firewall. Macs can connect to the server using an internal domain name like server.local domain, which points to the IP of the Mac Server. But, from time to time, users complain they cannot see the content of some folders and they need to reconnect or wait a few minutes until they can work again. Sometimes users also report that they don't see a file that other user put on the server a few minutes ago. It needs a network reconnect for the SMB/AFP to work again. These problems didn't happen before we split the network up in several VLANs - but the change shouldn't affect the connection between Macs and Server because they are in the same VLAN and subnet. The same problem occurs via SMB and AFP (so I think it isn't a problem directly with the network share ... but cannot tell it 100% for sure). I wasn't able to reproduce the issue. Everytime I tested it, it worked fine (via Wi-Fi or cable). **I got one chance to make some tests when a user that was experiencing the problem told me directly. I made some pings to the Mac server and it turned out that, when this happens, the ping to the server takes around 20 to 200 ms and sometimes packets are lost.** I don't know what the culprit is, because everytime I test it on different Macs or Windows/Linux computers everything works fine. Does someone have an idea where I can look for further troubleshooting or has someone had a similar problem and solved it? I would be very thankful for some hints that help solve this problem. Thank you for your help and best regards, Katharina