It seems like there are some problems with your database. We had a similar error message after installing a wordpress plugin that was not compatible with PHP5. In our case, the solution was deleting the plugin and erasing its entry in the options-database manually:
https://www.computerhilfen.de/info/wordpress-problem-catchable-fatal-error-object-of-class-__php_incomplete_class-could-not-be-converted-to-string.html (german, but with screenshots)
