I tried the <StrVal>, did not work.
This is from the Quest Map Item
<GameModifier>
<ModType>Map</ModType>
<Attribute>CreateQuestNearTile</Attribute>
<BoolVal1>1</BoolVal1>
<Value>-1</Value>
<Provides>Spawns a new quest</Provides>
</GameModifier>
I replaced the <BoolVal1> with <StrVal>. Not sure if <Value> means anything.