Problem adding computers to group

I have created some groups, and then wanted to add some servers to these groups. Most of them I had no problem adding, but some groups I get this error:

I dont really know what is causing this.

For some groups I tried renaming them, and suddenly I could add the servers, but for others this solution doesnt work

Note:  The following information was gathered when the operation was attempted.  The information may appear cryptic but provides context for the error.  The application will continue to run.
 
: Verification failed with [1] errors:
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Error 1:
: Failed to verify Discovery [UINameSpace140e4aac3df244f4ba437150a3d5b649.Group.DiscoveryRule]
Failed to verify referenced module : ID=GroupPopulationDataSourceInvalid configuration specified for Module [GroupPopulationDataSource]Incorrect expression : '$MPElement[Name="System.Mom.BackwardCompatibility.Library!System.Mom.BackwardCompatibility.Computer"]' specified. Unable to resolve this expression. Please check this expression for errors. (Hints:  Check for correct character casing (upper case/lower case), mismatched "$" signs, double quotes("), square brackets "[" or "]" ). Here is a sample expression: $MPElement[Name="Windows!Microsoft.Windows.Computer"]/NetworkName$
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Failed to verify Discovery [UINameSpace140e4aac3df244f4ba437150a3d5b649.Group.DiscoveryRule]Failed to verify referenced module : ID=GroupPopulationDataSourceInvalid configuration specified for Module [GroupPopulationDataSource]Incorrect expression : '$MPElement[Name="System.Mom.BackwardCompatibility.Library!System.Mom.BackwardCompatibility.Computer"]' specified. Unable to resolve this expression. Please check this expression for errors. (Hints:  Check for correct character casing (upper case/lower case), mismatched "$" signs, double quotes("), square brackets "[" or "]" ). Here is a sample expression: $MPElement[Name="Windows!Microsoft.Windows.Computer"]/NetworkName$
: Failed to verify Discovery [UINameSpace140e4aac3df244f4ba437150a3d5b649.Group.DiscoveryRule]
Failed to verify referenced module : ID=GroupPopulationDataSourceInvalid configuration specified for Module [GroupPopulationDataSource]Incorrect expression : '$MPElement[Name="System.Mom.BackwardCompatibility.Library!System.Mom.BackwardCompatibility.Computer"]' specified. Unable to resolve this expression. Please check this expression for errors. (Hints:  Check for correct character casing (upper case/lower case), mismatched "$" signs, double quotes("), square brackets "[" or "]" ). Here is a sample expression: $MPElement[Name="Windows!Microsoft.Windows.Computer"]/NetworkName$
: Failed to verify referenced module : ID=GroupPopulationDataSource
Invalid configuration specified for Module [GroupPopulationDataSource]Incorrect expression : '$MPElement[Name="System.Mom.BackwardCompatibility.Library!System.Mom.BackwardCompatibility.Computer"]' specified. Unable to resolve this expression. Please check this expression for errors. (Hints:  Check for correct character casing (upper case/lower case), mismatched "$" signs, double quotes("), square brackets "[" or "]" ). Here is a sample expression: $MPElement[Name="Windows!Microsoft.Windows.Computer"]/NetworkName$
: Invalid configuration specified for Module [GroupPopulationDataSource]
Incorrect expression : '$MPElement[Name="System.Mom.BackwardCompatibility.Library!System.Mom.BackwardCompatibility.Computer"]' specified. Unable to resolve this expression. Please check this expression for errors. (Hints:  Check for correct character casing (upper case/lower case), mismatched "$" signs, double quotes("), square brackets "[" or "]" ). Here is a sample expression: $MPElement[Name="Windows!Microsoft.Windows.Computer"]/NetworkName$
: Incorrect expression : '$MPElement[Name="System.Mom.BackwardCompatibility.Library!System.Mom.BackwardCompatibility.Computer"]' specified. Unable to resolve this expression. Please check this expression for errors. (Hints:  Check for correct character casing (upper case/lower case), mismatched "$" signs, double quotes("), square brackets "[" or "]" ). Here is a sample expression: $MPElement[Name="Windows!Microsoft.Windows.Computer"]/NetworkName$

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