Error in module when re-accessing main page

When main page access is allowed only for authenticated users, after second refresh (or hit) it gives an error:
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Fatal error: Call to undefined function: t() in /home/users/patryk/public_html/altust/modules/advuser/advuser.module on line 6
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When changing address to new one (like xxx.xxx.xx/something_new) it works, but for first hit as well.