Microsoft Edge Enterprise — Intermittent Blue Coat Proxy Authentication Prompt

Your screenshot shows a proxy credential dialog, not a Microsoft account sign-in prompt. The key missing fact is whether Blue Coat is actually advertising Basic, Negotiate, or NTLM when the interruption occurs. Microsoft documents repeated proxy challenges when routing or authentication exchanges fail; an intermediate 407 during Negotiate is not automatically a failure. Capture one occurrence with Edge’s network export facility and correlate its timestamp with the proxy authentication logs. Check effective ProxySettings and PAC routing for the triggering request, including background subresources, then have the proxy administrator confirm the offered scheme. If integrated authentication is intended, review AuthSchemes, the proxy hostname, SPN, and Kerberos tickets together. Do not widen authentication allowlists indiscriminately or enable Basic authentication over an unprotected HTTP connection as a workaround. Firefox behaving differently is useful comparison evidence, but does not alone prove an Edge defect.