What causes Teams won't sign in / connect?

Cached credentials / token corruption (most common) • WebView2 runtime missing or outdated (New Teams hard requirement) • Conditional Access policy targeting 'Microsoft Teams' cloud app • Network issue blocking specific Teams endpoints (signalling 443, media 3478-3481) • Tenant-wide service incident

How do I resolve Teams won't sign in / connect?

Determine scope (single user, group, all) • Test web vs desktop — narrows to client or identity • Clear cache / repair WebView2 if client • Address CA / network / service incident if identity or backend

How do I prevent Teams won't sign in / connect?

Standardise on 'New Teams' across the estate post-Classic retirement • Auto-update / WebView2 patching included in standard maintenance • Network egress policy aligned to MS 365 endpoint requirements • Avoid CA policies targeting Teams without exclusions for failure-modes

P2 · Microsoft 365 & Collaboration

Teams won't sign in / connect

Teams desktop client failing to authenticate or load — users blocked from meetings, chat, calls. Desktop and web behave differently and that helps diagnose.

Indicators

Teams desktop stuck on sign-in screen
Teams web works but desktop doesn't (or vice versa)
Status badge red, presence not updating
Recent migration to 'New Teams' (post-Classic Teams retirement)
Conditional Access blocking specifically the Teams app

Likely causes

Cached credentials / token corruption (most common)
WebView2 runtime missing or outdated (New Teams hard requirement)
Conditional Access policy targeting 'Microsoft Teams' cloud app
Network issue blocking specific Teams endpoints (signalling 443, media 3478-3481)
Tenant-wide service incident

Diagnostic steps

Test Teams web (teams.microsoft.com) in private window — isolates desktop client from identity issue
Check Service Health for Teams incidents
If desktop only: clear Teams cache (close client, delete %appdata%\Microsoft\Teams or %localappdata%\Packages\MSTeams_8wekyb3d8bbwe\LocalCache for new Teams)
Verify WebView2 runtime present and up to date
Review Sign-in logs in Entra — was the auth attempt blocked by CA?
Test Teams network endpoints — Microsoft 365 Network connectivity test

Resolution path

Determine scope (single user, group, all)
Test web vs desktop — narrows to client or identity
Clear cache / repair WebView2 if client
Address CA / network / service incident if identity or backend

Prevention

Standardise on 'New Teams' across the estate post-Classic retirement
Auto-update / WebView2 patching included in standard maintenance
Network egress policy aligned to MS 365 endpoint requirements
Avoid CA policies targeting Teams without exclusions for failure-modes

Tools

Teams web (browser-based)
Microsoft 365 Service Health
Entra Sign-in logs
Microsoft 365 Network connectivity test
Teams diagnostic logs (Ctrl+Alt+Shift+1)

References

Microsoft Learn — Teams troubleshooting / known issues
Microsoft Learn — Microsoft 365 endpoints reference

teamsmicrosoft-365conditional-accesswebview2endpoints