Hi, Although a newbe here I would like to reply with the details of my own quest to resolve this problem:
I have three sites, two hosted (linux) and one running on my own server (apache windows2003). When I started trying out VM ~ 3 weeks ago I installed version 1.1.0 on Joomla 1.5.6. No problems here everything worked fine.
My next step was installing it on my 2nd site. It had joomla 1.5.9 and VM 1.1.3 (2-3 days ago). I got the operation aborted problem both in front and backend. Tried almost all solutions here on this forum and only backend problem was solved. Firefox by the way = OK.
So I installed on my local machine J1.1.9 and VM 1.1.3.. Also operation aborted errors. Next I downgraded the VM 1.1.3 on the linux machine to 1.1.2, 1.1.1 and even 1.1.0. Always the operation aborted error popped up. Next, downgrading Joomla to 1.5.6.. Still that darn operation aborted problem. Tried the same thing on my local machine.. same error. So I copied the whole joomla install from my only working machine and.. tadaa NO ERROR.

(tested on both linux and local). So there must be a differences between the OLD 1.5.6 and the one I downloaded (for the downgrade). I even updated VM to 1.1.3 without any problems!

My idea is that somewhere in the core of Joomla something happened. (I don't know what). I should run a diff or so. (anyone interested?)