IBM i Access for Web java error with 5250

Hi folks

We migrated to a new 720 running 7.1 last year which meant our existing
Access for Web setup needed upgrading as well. Previously that had been
using the open source version of Apache Tomcat as the server.

On the new system we are using the integrated application server and on our
development LPAR all the Access for Web functions work fine. On our live
LPAR (with the same 5761JV1 licensed programs installed) everything except
for 5250 (works fine. For 5250 you just get

Error 500: java.lang.NoSuchMethodError:
com/ibm/as400/webaccess/servlet/e5250/ActiveSession.startSession(ZILjava/lang/String;Ljava/lang/String;Ljava/lang/String;)Z

as soon as you click on the Start session button. The log files report the
java application stack after that error, but nothing else.

Has anyone seen the same issue, or have any ideas where I can look next -
Google hasn't brought up anything meaningful so far (lots of entries for
that error, but not in relation to 5250)

Regards, Martin