That's certainly an odd error, but you probably need to nest the tabPanel inside both "<f:view>" and "<a4j:form>". The tabPanel is in the form, which is in the view, which is in the body. The form may not be required for this, but the view probably is.
form isn't nessesary..
but add f:view. And use <h:outputText value="Here is tab #1"> instead of plain text.
It's working now. The <h:outputText> did it. Thank you very much. Oddly enough though, when I reopened the project, the <f:view> tag was there, though I distinctly remember that not being enough to make it work earlier. Oh well. Thank you again for your help.