At some random point after working with the program for a couple of hours it won't let you save anymore and it won't "undo" a step anymore. I have seen no link between something I did with the program and the reoccuring of the problem.
I reset the preferences which solved the problem temporarily, but after a couple of minutes I was again unable to undo a step or save.
Of course, I can export the document as a pdf and thus create a copy of it so that my work is not lost, however, that's not a real solution.
The version we use is Adobe InDesign CS5 on Mac OS X. I can give you the exact details if you'd need them.
What can I do to solve the problem - or where exactly does the problem lie?
Is it only with the keyboard shortcuts, have you tried using the Menu Items, I know it sounds simple and apologies if it's too simple, but are they grayed out in the Menus?
I only ask because it happens me periodically - I have to go to File>Save and then everything works as normal.
Hold down the Cmd Key while choosing About InDesign from the InDesign Menu. What is the full version number listed in the upper left? It should be 22.214.171.1243. If it isn't, run Help> Updates... and if that says you are up to date, then you'll have to download the patch manually from http://www.adobe.com/support/downloads/detail.jsp?ftpID=5004
Do you have any plugins installed in InDesign that are not from Adobe. Plugins perform specialized tasks or extend the capabilities of the program. Many organizations like newspapers and magazines use specialized plugins to handle ad placement and tracking, for example. Font managers also tend to install auto-activation plugins so that fonts are loaded automatically if they are not already installed when a file is opened. These can be a big source of problems.
Other software not related to ID can also interfere with proper operation. We see reports of trouble with MacKeeper and similar utilities. Do you have anything like that running?