Directly after the feature has been activated you find the following message at each of the SPXXL Taxonomy Extension's WebParts:
Web Part Error: A Web Part or Web Form Control on this Page cannot be displayed or imported. The type could not be found or it is not registered as safe.
This a Sharepoint bug that'll occur after creating a new Web Application sharing the same Application Pool with another Web Application. Some modifications for registering DLLs safely are not copied in this scenario. As a workaround edit the web.config from
<%IIS-Root%>\wwwroot\wss\virtualdirectories\<%Port of Web Application>
and append a new Element to the Configuration/Sharepoint/SafeControls-Element:
<SafeControl Assembly='SharePartXXL.Taxonomy, Version=1.0.0.0, Culture=neutral, PublicKeyToken=c4dd89bc95ebaacf' Namespace='SharePartXXL.Taxonomy.WebParts' TypeName='*' Safe='True' />
(If used, DLLs for external Categorizers will have to be registered seperately, please ask us support@sharepartXXL.com)