Now, the last login we registered was on May 17. Have they used MISys since then?
If so, something changed on that day or the next day with how their security is setup and we haven’t registered any subsequent logins. I’d look more into any changes on May 17 or May 18 something has changed that likely is causing the issue you’re now running into. This is likely due to some setting with could be due to a security issue with your Network Connection, perhaps a setting on the proxy server (if you use one), etc. Or, try looking firewall/AV programs that could be blocking this.