Thanks , that's the hunch that I had as well.
Nothing useful in verbosed vpxd unfortunately:
2022-03-03T09:24:40.167Z verbose vpxd[04518] [Originator@6876 sub=Default opID=6c3ef3fd] [VpxVmomi] Invoking [initiateFileTransferToGuest] on [vim.vm.guest.FileManager:guestOperationsFileManager] session [52c9419b-aad6-6193-f4f7-3bb9a401edc9(528bdd1e-f724-7820-fb10-7daeba8ca431)]
2022-03-03T09:24:40.167Z info vpxd[04518] [Originator@6876 sub=vpxLro opID=6c3ef3fd] [VpxLRO] -- BEGIN lro-54326869 -- guestOperationsFileManager -- vim.vm.guest.FileManager.initiateFileTransferToGuest -- 52c9419b-aad6-6193-f4f7-3bb9a401edc9(528bdd1e-f724-7820-fb10-7daeba8ca431)
2022-03-03T09:24:40.167Z verbose vpxd[04518] [Originator@6876 sub=guestops opID=6c3ef3fd] [GuestOperationsManagerMo::PreProcessingModule] preprocessing vim.vm.guest.FileManager.initiateFileTransferToGuest
2022-03-03T09:24:40.167Z verbose vpxd[04518] [Originator@6876 sub=StateLock opID=6c3ef3fd] Attempting to lock vm-XXXXXX in mode SHARED
2022-03-03T09:24:40.167Z verbose vpxd[04518] [Originator@6876 sub=StateLock opID=6c3ef3fd] Successfully locked vm-XXXXXX in mode SHARED
2022-03-03T09:24:40.168Z verbose vpxd[04518] [Originator@6876 sub=proxyManagedObjectImpl opID=6c3ef3fd] Invoking vim.vm.guest.FileManager::initiateFileTransferToGuest with 6 params
2022-03-03T09:24:40.187Z verbose vpxd[04518] [Originator@6876 sub=Vmomi opID=6c3ef3fd] [ClientAdapterBase::InvokeOnSoap] Invoke done (<ESX_NAME>, vim.vm.guest.FileManager.initiateFileTransferToGuest)
2022-03-03T09:24:40.187Z verbose vpxd[04518] [Originator@6876 sub=guestops opID=6c3ef3fd] [GuestOperationsManagerMo::PostProcessingModule] postProcessing vim.vm.guest.FileManager.initiateFileTransferToGuest
2022-03-03T09:24:40.187Z verbose vpxd[04518] [Originator@6876 sub=guestops opID=6c3ef3fd] [GuestOperationsManagerMo::PostProcessingModule] postProcessing FileTransferToGuest, new url is 'https://<IP>:443/guestFile?id=6312&token=<TOKEN>'
2022-03-03T09:24:40.188Z verbose vpxd[04518] [Originator@6876 sub=guestops opID=6c3ef3fd] [PostGuestOpEvent] reporting event for InitiateFileTransferToGuest
2022-03-03T09:24:40.188Z verbose vpxd[04518] [Originator@6876 sub=Vmomi opID=6c3ef3fd] Invoke done: vim.vm.guest.FileManager.initiateFileTransferToGuest session: 52c9419b-aad6-6193-f4f7-3bb9a401edc9
2022-03-03T09:24:40.188Z info vpxd[04518] [Originator@6876 sub=vpxLro opID=6c3ef3fd] [VpxLRO] -- FINISH lro-54326869
VMware support said they don't cover these type of issue.
If you have any other tips they are greatly appreciated of course.