Hello,

I use code bellow and have a same error in two different SandBoxes.
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Component 
<aura:component >
    <aura:attribute name = "IsSunday" type = "Boolean" default="false"/>
    <aura:attribute name = "IsMonday" type = "Boolean" default="false"/>
    <aura:attribute name = "IsThuesday" type = "Boolean" default="false"/>
    <aura:attribute name = "IsWednesday" type = "Boolean" default="false"/>
    <aura:attribute name = "IsThuerday" type = "boolean" default="false"/>
    <aura:attribute name = "IsFriday" type = "boolean" default="false"/>
    <aura:attribute name = "IsSaturday" type = "boolean" default="false"/>
    <aura:attribute name="today" type="Date" default=""/>
    <aura:handler name="init" value="{!this}" action="{!c.doInit}"/>


    <aura:if isTrue="{!v.IsSunday}" >
       <p> Today is Sunday </p>
    </aura:if>
    <aura:if isTrue="{!v.IsMonday}" >
       <p> Today is Monday </p>
    </aura:if>
    <aura:if isTrue="{!v.IsThuesday}" >
       <p> Today is Thuesday </p>
    </aura:if>    <aura:if isTrue="{!v.IsWednesday}" >
       <p> Today is Wednesday </p>
    </aura:if>    <aura:if isTrue="{!v.IsThuerday}" >
       <p> Today is IsThuerday </p>
    </aura:if>    <aura:if isTrue="{!v.IsFriday}" >
       <p> Today is Friday </p>
    </aura:if>    <aura:if isTrue="{!v.IsSaturday}" >
       <p> Today is Saturday </p>
    </aura:if>
    
</aura:component>

and as a controller 
({
      doInit: function(cmp) {
        var Aujourdhui = new Date();               
          if  (Aujourdhui.getDay()==0) {cmp.set("v.IsSunday", true); }    
          if  (Aujourdhui.getDay()==1) {cmp.set("v.IsSunday", true);} 
          if  (Aujourdhui.getDay()==2) {cmp.set("v.IsSunday", true); } 
          if  (Aujourdhui.getDay()==3) {cmp.set("v.IsSunday", true); } 
          if  (Aujourdhui.getDay()==4) {cmp.set("v.IsSunday", true); } 
          if  (Aujourdhui.getDay()==5) {cmp.set("v.IsSunday", true); } 
          if  (Aujourdhui.getDay()==6) {cmp.set("v.IsSunday", true); }

      }
})


Thans for help
Hervé Colsenet