That's rather unusual, I'd highly advise double-checking the ports and possibly setting WireGuard tunnel MTU to a lower value on both ends (1380 for example).
If that doesn't help, could you please generate a troubleshoot file and send it over to me via private message? I'll leave instructions regarding how to generate the troubleshoot file as well as brief description about what it is below.
What's a troubleshoot file and how to generate it?
A Troubleshoot file contains the device's event logs, configuration files and other information useful for diagnostics. It can be downloaded from your device's WebUI, Troubleshoot page:
System → Administration → Troubleshoot