Comment # 12 on bug 1208659 from Fabian Vogt

If the "Error: cannot change the containment to AppletsLayout" messages
indicate the cause that looks like some applet is configured as containment
twice somehow.

I see in the config file that it's filled with some obsolete monitor
configurations, but that shouldn't be a cause.

I suggest you file a bug report upstream and attach the broken config there,
mentioning that it worked fine with 5.26.