Since version 17 of TIA Portal, compatibility issues have occurred with VPN client software. When starting TIA Portal on a computer with FortiClient VPN or OpenVPN client software installed, it goes into a BSoD (Blue Screen of Death) state, i.e. it crashes.

There seem to be several ways to resolve the issue, but personally, I have ended up deselecting the target SIMATIC Industrial Ethernet (ISO) from the Fortinet SSL VPN Virtual Ethernet Adapter interface settings on my computer, as shown in the image below.

This has fixed the issue for now.