CS5.x ist not supported on Mac OS X 10.11 as it is incompatible. You will have to subscribe to Creative Cloud.
That cannot be the case as I have been using ID CS5.5 without problems prior to the ID update! Also Photoshop CS5 works.
You are wrong. Even if it is running for a while, it does not mean, that it will always work. If it was working for a while, you were happy for a while, but if it will not work anymore, you are alone with that old version, as it is not supported officially. Only small changes in the OS cause huge problems in not supported programs.
Is there a way I can go back to the previous version of ID CS5.5 without doing a complete reinstall (I don't have the license at the moment)?
Seems like I will eventually go back to OSX 10.9 (that's the version I upgraded from), but I really need a faster solution for the moment. Is CS.5.5 compatible with OSX10.10?
Paying for CC is not an option at the moment as I already own both CS5.5 and Lightroom 6 and they are fine for my needs.
You can download the installer from here: Download Adobe Creative Suite 5.5 products
Whether or not those are patched releases or not, I don't know but I would doubt it.
There's no assurance it will work on 10.11, though and I would back up to 10.9.
Many thanks for your advice!
I finally just reverted to Mavericks. ID now works, but it gives an error message: "Cannot load Assignment UI.InDesignPlugin because it requires InCopy Bridge. Please install the InCopy Bridge plug-in and restart InDesign". I think I've had the same error before, but I can't remember how I got rid of it. Maybe I just ignored it as I don't use Bridge anyway.
How did you revert? Adobe applications don't do to well in many recovery operations.
I'd run the CC Cleaner tool and reinstall it.