[Mailman-Users] Bug when editing Mailman html pages

[PeteBell]

I have just tried to edit the user-editable html pages (subscribe,  
option results, list-info, and the 'welcome to new subscriber' text  
message), but I get the following message...


Bug in Mailman version 2.1.6
We're sorry, we hit a bug!
Please inform the webmaster for this site of this problem. Printing  
of traceback and other system information has been explicitly  
inhibited, but the webmaster can find this information in the Mailman  
error logs.


I stress that I have edited these pages successfully before, but all  
of a sudden it won't work.

Any ideas please?

TIA
PeteBell, UK