This example Crash Dives, I've tested it and causes the exception.

First you'll need a new Custom Settings called "Products__c", type of the Custom setting is LIST and it'll need only one instance called "General" and one field (string 15) called RecordTypeIdPack__c with one of your Quote Id's

Probably you could mimic this without creating the custom setting, but I included it for the example to be as accurate as possible.

Next you'll need this page:

<apex:page standardController="Quote" extensions="ProductWizardERRORTEST" sidebar="false" tabStyle="Quote">
	<apex:form >
		<apex:pageblock id="SearchBlock" title="Search Block" mode="detail" rendered="{!ShowSearchForm}">
			<apex:selectList id="selectTipo" value="{!SearchType}" size="1">
				<apex:selectOptions value="{!lSelectType}"/>
	        		<apex:actionSupport event="onchange" action="{!EvaluateVisibility}" rerender="SearchBlock"/>
			</apex:selectList>
		</apex:pageblock>
	</apex:form>
</apex:page>

 And the Class behind:

public class ProductWizardERRORTEST {
	// Properties for the select list
	public boolean ShowSearchForm {get;set;}
	public Id SearchType {get; set;}
	public list <SelectOption> lSelectType {
		get{
			if (lSelectType == null){
				list <SelectOption> lSelectTypeAux = new list<SelectOption>();
				List<RecordType> lRT = [SELECT Id,Name FROM RecordType WHERE SobjectType='Product2'];
				for (RecordType rt :lRT) lSelectTypeAux.add(new SelectOption(rt.Id,rt.Name));
				return lSelectTypeAux;
			} else return lSelectType;
		} set;
	}
	// Product record types custom settings Initialization
	Products__c ProductConfig {
		get{
			if (ProductConfig == null)
				return Products__c.getInstance('General');
			else 
				return ProductConfig;
		}
	}
	
	// Constructor
	public ProductWizardERRORTEST(ApexPages.StandardController controller) {
		ShowSearchForm = true;
	}

	// Method that crashes
	public void EvaluateVisibility(){
		// THE ERROR TRIGGERS IN THE NEXT LINE
		if (SearchType == ProductConfig.RecordTypeIdPack__c){
			ShowSearchForm = false;
		}
	}
}

Now open the page and change the value of the picklist and... BAM!

Salesforce System Error: 1935329124-80909 (1564369920) (1564369920)

Se ha producido un error inesperado. Se ha notificado a su organización de desarrollo.

I hope that helps.

*** EDIT ***

I forgot to say  that if you change the ID property to String, then it doesn't fail:

public String SearchType {get; set;}