Based on your description, there are a few possible causes for why Internet Explorer is still showing an error message despite having tried various troubleshooting steps:

1. Incompatible ActiveX components: Although you mentioned that all relevant settings were enabled or set to 'prompt', it's worth double-checking the specific ActiveX components required by the application and ensuring they're up-to-date.

2. Corrupted user profile: Sometimes, a corrupted user profile can cause issues with certain applications. Try creating a new local admin account on your system (this will delete all of your personal settings), log in as this new administrator account, then try opening Internet Explorer again to see if the problem persists. If it doesn't, you'll know that there's something wrong with either your user profile or one of its components.

3. Malware infection: It might be worth running a full scan using an anti-virus program like Norton Antivirus or Symantec Norton AntiVirus 2005 (the latest version) to rule out any potential malware infections which could be interfering with Internet Explorer's functionality.

4. Corrupted system files: System file corruption can sometimes cause application-related issues, including those related specifically to web browsers such as IE11. You may want to try running the 'sfc /scannow' command from an elevated Command Prompt window (right-click on Start > select 'cmd.exe' and hold down Shift while double clicking it) followed by a subsequent restart of your computer after its completion.

5. Registry issues: Sometimes, problems can arise due to registry entries becoming corrupt or mismatched between different versions installed side-by belt without proper removal prior before installing newer ones overtop them which could lead into conflicts especially when trying out beta releases etcetera...

Microsoft catalog internet explorer 10