First, it could be a network issue, either very high latency or a brief (but complete) lack of connectivity. To fix this is a matter of making sure the connection is in good working order.
Second, in some cases, a filtering service such as DNSFilter could have the URL used by SaaS Alerts service categorized as a ‘Parked site/domain’, which is blocked through your filtering policy. The address is the-byway-248217.firebaseapp.com. If you approve this you should be able to get in.
Third, certain Anti-virus software (web proxy) will use browser protection or locking options that prevent SaaS Alerts from loading correctly. App examples: Todyl, Bitdefender, Sophos, and HP Security. You will want to disable this feature on any browser being used to access SaaS Alerts so that it functions as intended.
A good first troubleshooting step is to see if you are able to log in to manage.saasalerts.com using a cellphone or tablet not connected to your internal network (5g/or other connection not part of your wifi)
Each Network security appliance/application will have a different place to add these exceptions.
Sophos for instance would need to be added via Endpoint protection -> threat protection -> settings -> exclusions and add the above sites there.
You may also want to consult the article: Cannot Connect Application