tail -f EssbaseLog.txt
will run forever and will present lines as they are added to the file. Thus your script will run forever too. You run your script once and let it find errors as they occur.

You didn't use --follow=log-file but you may need it. Post the output from:
uname -a
so folks know what version of unix you have.

ReadLines=0
while [ true ]
do
TMPVAR=$(date +%s)
Totallines=$(wc -l /var/log/YOU.log )
Totallines=$((Totallines-ReadLines))
tail -n $Totallines /var/log/YOU.log | grep -A 50 'PATTERN' > /tmp/$TMPVAR.tmpfile
if grep 'username' /tmp/$TMPVAR.tmpfile ;
 then
   mail -s failure xxxxx@yyy.com < /tmp/$TMPVAR.tmpfile
fi
rm -rf /tmp/$TMPVAR.tmpfile
sleep 900
done

1. Use crontab to run every fifteen minutes, key "man crontab" in shell.

2. This script may work <script.sh <your text to grep>:

#!/usr/bin/ksh

thelog="/your/local/logpath"
user=$1
lines="50"

run () {

num1=$(cat -n $thelog |grep $1 |awk '{print $1}' |head -1)
num2=$(expr $num1 - $lines)


if [ $num1 -gt 50 ]
then
results=$(cat -n $thelog |sed -n "$num2,${num1}p")

else
results=$(cat -n $thelog |sed -n '1,50p')

fi
echo "$results" |mailx -s "blabla" yourmailbox@nospamplease.com
}


run $user;