Shared folder or PC not found in the network?

I bought an additional PC with WIN10. This does not find the shared folder on the "main PC"

It can be found on my laptop.

Connected via LAN cable to the FritzBox.

The main PC finds the "new one"

Is the network in the PC set as a "private network" or as a "public network"? You can only see the other computer with the setting "Private network". However, it is imperative that all PCs are in the same workgroup or domain

The usual suspects are all right.

private network
WORKGROUP

… It works the other way around. Only my "main PC" locks itself against this one new PC. He gets along very well with everyone else.

Are all the settings of the "new" PC identical to those that are already communicating with the main PC (firewall, network, etc.)?
Somehow the NEW is configured differently, if only the firewall would block that.

Can you ping the main PC?

Ps: can also be due to the setting of the Fritzbox. You can define a port as a guest access, maybe that's exactly the one where the new computer is attached