It sounds like a permissions error to me. Have you recently changed permissions on moved anything in your /domains/548/web/tb/uploaddata/ directory? The filesize function returns that warning when the directory is not writeable.
If that doesn't help, you can try running the TeX diagnostics which will be at
http://your.moodle.site/filter/tex/texdebug.php
(obviously modify for your own site and you'll need to be logged in as admin).
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Text notetion error
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