As a workaround, you might be able to fix it by forcing removal of the Archivist and Taglister packages, then downloading and reinstalling them. That way they should have valid source values.
BTW, I just helped another user on whose site the Archivist and Taglister packages were marked as installed in Package Manager but were not actually installed.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html