UPDATED: made some wrong assumptions about classes etc. The following occurs now when I have a 'demo' project:
I have two classes, both named 'Company'.
- One is placed in grails-app/domain/my.classes.domain.Company
 - the other is in src/groovy/my.clazz.Company
 
The last one has a @Validateable annotation, and the Config.groovy contains grails.validateable.packages = ['my.clazz']
I also have an Account class, in grails-app/domain/my.classes.domain.Account:
    package my.classes.domain
import java.io.Serializable;
class Account implements Serializable { Company company }
Then I use the following code (in the bootstrap.groovy):
import my.classes.domain.Company
import my.classes.domain.Account
...
Company company = new Company
Account acccount = new Account(company: company)
When running this app, the following error is shown:
Caused by: org.springframework.beans.ConversionNotSupportedException: Cannot convert value of type [my.clazz.Company] to required type [my.classes.domain.Company] for property 'company': no matching editors or conversion strategy found
    ... 33 more
Caused by: java.lang.IllegalStateException:
Cannot convert value of type [my.clazz.Company] to required type [my.classes.domain.Company] for property 'company': no matching editors or conversion strategy found
This is a very strange exception, as everything seems to be fine. Some testing proved the following 'hints': This error does NOT occur when I modify the config.groovy to explicitly name the classes (i.e. use grails.validateable.classes = ['my.classes.domain.Company']), This error does NOT occur when I modify the Account's company property to be named differently (and modify the bootstrap accordingly), i.e.:
class Account extends Serializable {
    Company cmp
}
However, these are workarounds. I'm really interested in WHY this is happening. Anybody got a clue?
Just to be on the safe side, I did the following to create this problem:
- create domain class: my.classes.domain.Company
 - create domain class: my.classes.domain.Account
 - modify the domain class as above
 - create a groovy class: my.clazz.Company
 - give this groovy class the Validatable annotation.
 - add the my.clazz package to the validateable packages
 - in the bootstrap, create a new Account with new Account(company:company)