$this->state is treated as array.
However, if there was some error (i.e., import was interrupted in the middle, failed uprade from D6 etc.),
load() sets $this->state either to false, or to string.
In the latter case attempt to unset array element fails with
PHP Fatal error: Cannot unset string offsets in .../sites/all/modules/contrib/feeds/includes/FeedsSource.inc on line 348
attached patch fixed the problem by explicitly checking type of unserialized $this->state