Truclient getting 'dead object' errors

I have a number of Truclient scripts (12.60) which perform multiple iterations per login (ie every n'th iteration user logs out & back in). Forced logouts also occur when the script encounters an error. When doing this logout I am seeing the following error:

t=01819792ms: Error -205177: TypeError: can't access dead object
instanceofDate@resource://gre/modules/ExtensionContent.jsm:5118:11
serialize@resource://gre/modules/ExtensionContent.jsm:5268:10
serialize@resource://gre/modules/ExtensionContent.jsm:5296:19
serialize@resource://gre/modules/ExtensionContent.jsm:5328:16
toJson@resource://gre/modules/ExtensionContent.jsm:5357:15
sandboxToString@resource://gre/modules/ExtensionContent.jsm:5621:11
ArgsContextToString@resource://gre/modules/ExtensionContent.jsm:8644:24
_storeState@resource://gre/modules/ExtensionContent.jsm:8671:21
onUnload@resource://gre/modules/ExtensionContent.jsm:8741:5
onload@aumelvperfpc05/.../wfrmLogout.aspx : Failed to serialize array. [MsgId: MERR-205177]

Since I am doing multiple iters per login, the 'new user per iteration' flag is turned off, which I suspect is part of the problem. I should run the script with the flag on and one iter per login to test this theory, but simply have not got there yet.

I haven't been able to find any relevant hits for 'truclient dead object' either here or in Google so I'm either doing something very tricky, or very stupid. The error does not seem to be fatal, the vuser continues on its way, but it sure fills up the logfile.

Appreciate any insights.

Steve