Hello Fritz,
Thank you for your message.
I've checked on this issue with our developers - they've confirmed that the issue is caused by some security software, i.e. antivirus/firewall software or even router. In this case, the issue is most probably caused by SentinelOne antivirus software that you've mentioned earlier.
In order to confirm this, you can try uninstalling the antivirus software from one of your machines and then try launching Portable Viewer of version 7.0.0.3 once again to see if the issue still persists. In case if Portable Viewer of the latest version works this way, then we recommend that you submit a false positive to the SentinelOne antivirus as well.
Please let us know if you have more questions.
