I have used sticky blocks in the past with no problem, last night at home and again this morning at work i have had this error message.
print_header() was called more than once - this should not happen. Please check the code for this page closely. Note: error() and redirect() are now safe to call after print_header().
the sticky block operates ok but will not allow me to create links from images. if i use a text link then there is no problem.
these may be two entirelty separate problems but i don't know enough about moodle/php etc. to be able to comment.