we are trying to associate a new Flex device with our Sandbox account but inputting its SN and clicking Associate Device results in a couple of 404 errors. Furthermore, it is impossible to select anything in the Test Merchant dropdown because there are no options. The list is simply empty even though there are Merchants set in the system and three other devices are associated with them (these associations happened in the past).
These are not the only problems we're having with the Sandbox - it's currently also impossible to access Account Settings (this happens in the EU production environment too) and also we get logged out way too frequently without any apparent reason.
Depending on the Role assigned to you when invited by the Main Account, you are granted different permissions. To attain the permissions necessary to interface with the account as you described, have the Main Account edit your Role to "Admin."
Furthermore, you can only see another Member's Test Merchant if they have added you as an Employee to their Test Merchant.
As far as associating a new device, only the account who created the Test Merchant can associate devices to said Test Merchant.