What version of Mattermost are using, and could you post a screenshot of the exact error message? I thought the diagnostics code fails silently if there's no internet connection, but I could be wrong about that.
If it is the diagnostics causing the errors though, they can be disabled by turning off these settings in the System Console:
- General > Logging > Enable Diagnostics and Error Reporting (
LogSettings.EnableDiagnosticsin config/config.json) - Notifications > Email > Enable Security Alerts (
GeneralSettings.EnableSecurityFixAlertin config/config.json)