Field type xxx is not installed properly. Go to the list settings page to delete this field.

A little reminder for myself (and all the other custom -field-developers) how the notation of the value for the FieldTypeClass-attribute in the fldtypes_xxx.xml has to be to prevent the nasty error message like this posting is titled:

<field name="FieldTypeClass">namespace.fieldclassname, assembly name, Version=versionnumber, Culture=culture, PublicKeyToken=publickeytoken</field>