Workbooks.Open (Readonly:= true) fails at random times

Hello guys(and girls)
this is my first post and i really don't understand why my code isn't just working.
well it works, but when it runs for some minutes or hours it stops because of the ignoring popup to insert password of the file i want to open.. but i only need to open it in readonly mode.
Is there here someone who knows what is going on? om working with excel 2010

this code is the problem:
Set wbSource = Workbooks.Open(Location & "\" & Filename, ReadOnly:=True) ' should open file in read only mode so no password popup pops up

hope there is someone who can help me.

greetings, WannabeNerd

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some extra info:

when i use workbooks.open (Location & "\" & Filename),WriteResPassword:="Password"

the popup (Password File.xlsm is reserved by enter password or open read only) pop ups also at random times.. it's working fine for minutes/hours, but then at random it fails....