Could be an issue with some specific fonts or the Adobe font managment in general. If you start up PS first, it seems to lock up one of the fonts needed for the application UI and AI can't access it. Could be permissions, could be a bug, could simply be legacy software on new systems not working as expected. Hard to tell...
Can I make a sincere suggestion?
Glad you said yes!
How about upgrading both your software and hardware, is there any chance you can do this? Eventually you will have to.
You might find this to offer you not ny a solution to your current problem but also something that will make you much more productive.
It seems to be connected to the ADMUI3.fon file.
For a still unknown reason illustrator is trying to open the ADMUI3.fon file from the photoshop install directory and is denied access.
Even though illustrator has the ADMUI3.fon file in its own install directory.
Thanks for your replies, still looking into the problem.
I will give your suggestions a try, great info about the aiprefs file.
I have discovered some more info:
When photoshop is running, illustrator tries to create/edit/read the ADMUI3.fon file inside the photoshop install directory (on startup).
This fails because the virtualized instances of photoshop and illustrator don't have any rights in each others dirs.
When photoshop is not running, illustrator doesn't try to create/edit or use the ADMUI3.fon in the photoshop dir, but instead uses its own.
Sure does sound like a corrupt preference.