Today I had a big surprise, several of my sites went out of business with the "Error: 500 - String could not be parsed as XML" message. I checked everything, and found no clues what can be happened. the sites weren't hacked, files or database has no signs to being tampered width or being damaged.
And usual bug-hunting tactics as unpublishing modules for example revealed also nothing.
After a while I realized, that one of common things these sites have was the Google Maps plugin. And a search revealed finally what's up:
Google in their infinite wisdom have changed their Google maps API... and I have using the Google Maps plugin (mosmap) v 2.*. Even if the plugin was set to use the Google Maps API version 3.* the reality check showed, that the plugin works only with the early version of Google Maps API, 3.0. Tried -to upgrade the plugin to the latest found on JED, and that fixed the problem. So, if you have the similar problem, upgrading the Google Maps plugin to v3.1 or later very probably will fix the problem.
It's pretty shame, that the old version not used the Joomla updater - and this way does not helped me to nail down the problem easier. Fortunately the new version of plugin uses it, so the future looks brighter!