Feel free to delete the data from session table (not table itself). It should not affect anything in Sitebuilder. BUT, just in case I can suggest you first make sitebuilder3 database dump:
On system with Plesk installed:
# mysqldump -uadmin -p`cat /etc/psa/.psa.shadow` sitebuilder3 > /root/sitebuilder3.sql
On standalone server:
# mysqldump -uroot -pPASSWORD sitebuilder3 > /root/sitebuilder3.sql
Let me know about the resutls.