Some uploads could fail because of a corporate proxy that is filtering traffic. The particular document might be matching a pattern that causes the proxy to deny the upload. These sorts of filters are used to stop malware and stop sensitive information from leaving the firm.
You can determine whether there is a corporate proxy in place by looking at the certificate issued for Safelink by clicking on the padlock icon next to the URL bar. The certificate will have an issuer. If the issuer is R3, it's likely correct, and there is no proxy. If the issuer is anything else, there is a proxy in play, and the firm's IT team can add an exception for the Safelink domain. Please get in touch with us if you need further assistance.