The hosts file change mentioned 4 months ago would be if there had been some hacking going on that disabled the Adobe activation server using the “127.0.0.1 activate.adobe.com” line in the hosts file and the adding of the # in front disables that hosts entry, effectively re-enabling the activation server access by programs on the computer. Unless your hosts file actually has the entry to disable the adobe activation server or other adobe servers, then your problem would be different. The error in general means that when you are right-clicking and opening in ACR, the host application for ACR, which is Photoshop, cannot start. One reason might be that the place the right-click Open in ACR command is looking for Photoshop doesn’t exist, anymore—maybe a registry or config-file has the wrong thing in it, and another reason would be the an existing version of Photoshop is being started up, but something went wrong before the startup completed. If you are on a 64-bit Windows system, there are potentially two Photoshop executables, one 32-bit and one 64-bit. With CS6, at least, there are also two versions of Bridge, so maybe it matters which 32-vs-64-bit Bridge you’re running. To test this try starting up the 32-bit Photoshop and right-click-Open-in-ACR, and if that doesn’t work, then shut down the 32-bit version of PS and start up the 64-bit version of PS, and try the Open-In-ACR again. If it still doesn’t work, then there is some other problem. Adobe applications can sometimes be confused if there have been trials and release versions on the same system that aren’t uninstalled and installed in the right order. There is a CS5 cleaner script you can search for on Adobe’s site that attempts to remove all traces of CS5 from your system. You would only want to run this after you’d uninstalled as much as you could, yourself.
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