Strange issue with updating our software

Hello,

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I work for a software development company and we have recently ran into an issue when sending updates to our customers.

To software we make is written in Java and we have recently switched to Installbuilder for delivering it. We are sending the updates through an auto-update feature that will ask for admin permissions, bring up a loader, connect to our server ( that is running Apache 2.4.29 ) and download the update. After the download is done, the update is installer and our software is launched automatically.

We have switched to the Installbuilder compilation method around 4 months ago and we keep having issues with about 0.5% of our customers that have trouble updating the software after we deploy updates. I have managed to remotely connect to one of our customers that was having issues with the update and he receives "Error connecting to server" messaged during the auto-update loader.

Being in need to fix the issue as the customer had to work in the program, I have accessed our website to download the installer ( that has the latest version ) but the customer couldn't download the installer from the website. It was showing "Starting…" in his Firefox browser but the download would never start.

I have tried to download something else from different websites and his download was working good, so the issue was with downloading from our server / website. I tried to download / update the software from other PC-s and everything was working good on our end. Also some other users were updating the program while I was on the phone with the customer.

The customer managed to update to each new version without any problem as we deployed many updates in the last few months but just couldn't update / download now. I have also tried to switch the browser but with no success.

I have checked his PC for antivirus , proxy , vpn , different browser settings but nothing helped.

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It is really odd that he could download from any website but not ours, even though the website was loading and showing all the info correctly.

This issue happened with a few other customers also and I really need to find a fix for it as 0.5% of users not being able to update is unacceptable.

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Did anyone ever encountered something like this ? I have tried searching on Google the issue but I couldn't find anything related. How can you download anything from the Internet but not from our website, along with the fact that he couldn't connect to our server to do the update.