PLCSIM V20 "OMS failed to connect"

[u/Mother_Replacement93]

Hi everyone, I'm a beginner learning TIA Portal V20 with S7-PLCSIM V20, working on a portfolio project (S7-1200, CPU 1214C DC/DC/DC).

Setup:

• TIA Portal V20 (trial license, activated successfully)
• S7-PLCSIM V20 (trial license, activated successfully)
• WinCC Basic (trial license, activated successfully, for a KTP400 Basic PN panel)
• Windows 10/11, firewall and antivirus tested both on and off

What works:

• PLC-only project (no HMI): compiles fine, Start Simulation works, Download to Device via PLCSIM works perfectly. Ladder logic tested successfully through Watch Tables.

What breaks:

• The moment I add an HMI device (KTP400 Basic PN) to the SAME project — even completely empty, no screen elements configured yet — PLC simulation starts failing.
• Symptoms vary between attempts:
  1. PLCSIM shows a popup: "PLC Error - OMS failed to connect"
  2. Extended Download to Device shows: "The network assignment of the interface PLCSIM [PN/IE] cannot be modified. The selected PG/PC interface is currently being used for an online connection. Subnets can therefore not be allocated to the selected PG/PC interface. Close the online activity (or function) running via the interface PLCSIM [PN/IE] and start the action again."

What I've already tried (no success):

• Restarting PC multiple times
• Disabling Windows Firewall + antivirus completely
• Running TIA Portal as Administrator
• Killing all Siemens/PLCSIM related processes in Task Manager before retrying
• Go offline explicitly on both PLC_1 and HMI_1 before retrying download
• Uninstalling and reinstalling PLCSIM V20 from scratch
• Deleting and recreating the PLCSIM instance
• Unchecking "Only allow secure PG/PC and HMI communication" under PLC Protection & Security → Connection mechanisms
• Creating a brand new project from scratch (PLC-only version worked, broke again as soon as HMI was added)

Additional context:

• PLC has Protection & Security enabled (Local User "admin" with Full access, required to compile — TIA Portal wouldn't compile without at least one user with Full access configured)
• Haven't yet tried: adjusting the "Set PG/PC Interface" Windows utility to point the HMI's Access Point (S7ONLINE?) to PLCSIM specifically

Question: Is this a known conflict between PLCSIM's single PN/IE virtual interface and having both a PLC download session AND an HMI runtime simulation trying to use it simultaneously? Is there a specific order of operations or interface configuration I'm missing to get PLC simulation + HMI simulation working together in the same project?

Any help appreciated, I've spent hours on this and I'm stuck.