The behavior you are experiencing is related to Bubble's systems not being equipped to handle wildcard subdomains. When navigating to a subdomain of a Bubble app that is not hosted on that subdomain, certain features of the app will no longer work and will throw errors. The features known to not work at the moment are:
Setting cookies
By extension, logging into the app
SSL / Certificate lookup; which means that all traffic to subdomains is sent over plain http with no security
The debugger and anything that requires checking app ownership / permissions
As a workaround, using URL parameters to accomplish the necessary actions may help, but currently there is no way to register an arbitrary subdomain to a bubble app.