Fatal Error EBR 2.0.8.1

Please help, I have an EBR entry and each time I try to publish it it crashes my whole website and displays:

Fatal error: Call to undefined function mb_substr() in /home/sarahsco/public_html/modules/mod_ebrlatestentries/helper.php on line 45

I'm running version 2.0.8.1 on Joomla 1.5.22

Is there a limit on the number of words/characters that can be entered in a review because if I take out some words it will publish OK. If the answer is 'yes' is there a way of increasing how many words can be published per review?

Thank you for taking the time to look at this.

zeebee

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