Icons missing in TinyMCE and HTML-Editor?

I just encountered the problem of disappeared buttons in the editor, both in HTML and in TinyMCE view – the latter I couldn’t even select. As I didn’t find the solution in the WWW, I now post it here to possibly help others:

The problem was the PHP memory usage. My installation is running close to the upper limit as given by the provider. Once I reduced the usage by deactivating the plugins I do only need for maintenance purposes, the editor worked fine again.

I crosschecked: It was not due to specific plugin conflicts, but only due to overal PHP memory usage.

Hope this might help others with similar problems.

Cheers
Anders

ps: I find the plugin “WP Memory Load and db size Usage” to be very useful as it shows the actual PHP memory usage in the admin bar.