[ic] 4.8.7 - Clicking "Design" now gives 500 error ?

[DB]

I have 4.8.7 using MySQL on Redhat 8. I've made no recent changes and 
the catalog seems to run ok. But if I click the Design tab in the UI I 
now get a Inetrnal Server (Premature end of script headers: 
/home/myuser/cgi-bin/mystore in apache error log)

In the interchange error log I see a line like:
  page server pid 25379 won't die!


Any ideas what has gone wrong?

DB