"Missing Feeds plugin FeedsREGEXParser"

I installed and enabled Feeds REGEX Parser 7.x-1.0-alpha1 on Drupal 7.10 that has Feeds 7.x-2.0-alpha4. When changing to use REGEX Parser in Feed importer settings, the setting didn't stay saved (none of the radio buttons is now checked) and I got the following instead:

Missing Feeds plugin FeedsREGEXParser. See [FEEDNAME]. Check whether all required libraries and modules are installed properly.
Missing Feeds plugin FeedsREGEXParser. See [FEEDNAME]. Check whether all required libraries and modules are installed properly.

Changed parser plugin.

Error message

Notice: Undefined index: FeedsMissingPlugin_feeds_form in drupal_retrieve_form() (line 750 of [DRUPALROOT]/includes/form.inc).
Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'FeedsMissingPlugin_feeds_form' not found or invalid function name in drupal_retrieve_form() (line 785 of [DRUPALROOT]/includes/form.inc).

[FEEDNAME] and [DRUPALROOT] stated the appropriate name and path instead.

This plugin is exactly what I need for a certain site and I hope it is possible to try it soon. The only thing better would be a plugin that allows to read an RSS feed as RSS, and to split the information in to be saved in different fields of a node. Feeds + Feeds_Tamper gets close, but instead of REGEX allows only REGEX replace, and doesn't really allow you to take one part of to field1 and a different part to field2.

Thanks for your work for the community, twistor.