In case less than 500 MB free memory are available, the overall performance of the system as well as the stability of single programs might be reduced severely.
On a 64-bit system, you should make sure to run the 64-bit version of your Steinberg program and additionally installed plug-ins.
The most common reason for USB-e Licenser (dongle, formerly named Steinberg Key), Soft-e Licenser and/or license related problems is an outdated or corrupted e Licenser Control Center installation.
So if an USB-e Licenser or a specific license on it is not recognized, or you get any license related error messages when starting your Steinberg sequencer, you should download and install the latest e Licenser Control Center at first.
In case your system is short of memory, you have the following options: If Cubase/Nuendo hangs or quits during the start up or while loading a specific project file it is likely that an installed VST plug-in is causing trouble.
Otherwise the program would try to comply with them when creating fresh preferences. Initialising the program's preferences does not necessarily mean that you lose all settings.
For example if you have defined your own set of key commands that you want to keep you can replace the corresponding file (Key Commands.xml) in the freshly created folder by a copy of the same file taken from the folder you've disabled by renaming.
Another approach: Since in many cases the file "Defaults.xml" will be the trouble maker you also could try if renaming or deleting only this particular file already solves the problem.
This Knowledge Base article provides further information on Cubase's preferences and their location: Cubase and Nuendo Program Preferences Files in Detail Making a backup of the preferences obviously is a good idea, especially when you customize a lot.