vClient-vServer communication is based on some "protocol". Passing "identifiers" is a part of that protocol.
Most of vClient's objects are some "stubs" paired to vServer's objects - "proxies". vServer's "proxies" are paired to vKernel objects.
So, most of the methods called in vClient looks like: "send all relevant data (like "my identifier" and arguments) to the vServer's proxy", and then, vServer's "proxy" calls appropriate vKernel object.

"An invalid identifier" means that vServer knows nothing about such a vClient "stub". Most probably the "proxy" was deleted in vServer, but vClient wrongfully use "dead" stub.

Thus, in terms of V4RB/Xojo, I can say vClient send API command to the vServer, whereas the connection is lost. Am i correct saying there is no invalid command in this case, but it's most probably a connection problem ?

No, it's ok for connection.

Most probably it is because of calling some "dangling objects". Try to search in vServer log (vServer must be run with verbose mode = 3) that address - 0x942D140. I think it is the stub for some cursor.
Anyway, your app receives an error from vServer and it is either caught in your app or should be a reason for app termination (crash).

Ok. Thank you.