After unsuccessfully installing Aptana (for RadRails) via Software Updates -> Find and Install… (failure error message), I manually copied the files provided by Aptana into the features and plugins directories and restarted MyEclipse. I got more error messages, so I manually removed those files before restarting again. Unfortunately, I now get the following error message the first time I go to Software Updates -> Find and Install…:
Requested operation cannot be performed because it would invalidate the current configuration. […] Platform configuration has been modified outside this program. A restart is recommended.
and this one all subsequent times, before restarting the IDE:
Another install/update job is in progress. Please cancel it and then re-launch this action.
Thanks for your help.