If the device does not have Web Content Filtering, VPN, or Global HTTP Proxy configurations applied through Hexnode, Hexnode is unlikely to be blocking SharePoint traffic.
An “Error 3” message in this context usually indicates that the app was able to send the request, but the app’s backend service returned an invalid or broken response. This points more toward an application-side or service-side connectivity issue rather than an MDM restriction.
To isolate whether a Hexnode policy is involved, you can test with one device by temporarily removing the associated policy, saving the policy changes, and then checking SharePoint search again. If the issue continues even without the policy, the next step would be to check with the app vendor or the internal IT team managing SharePoint/Microsoft 365 access.