Thanks. Setting the width and height did the job.
The other issue? Well, sometimes there is an advantage in accidentally overwriting a file: when rewriting the header and the footer templates the problem seems to have disappeared. Leaves me only with the problem of not knowing what I did wrong.
Probably something went wrong with the sequence in which stuff was loaded.
With the $(slider) and $(slider2) scripts hardcoded into the footer things do not work. Using a plugin (HiFi) to add the scripts to the footer works.