I went through the configuration of botch stacks page by page and I found out that the LLDP settings were not the same. I changed all the ports from AdminStatus "txAndRx" to "rxOnly" and voila, access to the VLAN was granted directly ;-).
I have no idea what I just did, but I guess I can worry about that later. For now, it seems to be working.
FYI, all four (two stacks) switches are Nortel 5520-48T-PWR. I am not a network engineer, so forgive me if my question was silly ;-)
Greetings and thank you all.