I've also had occasional problem with lost worksets (no errors given). I've notiiced that lost was more possible during open operation and was less possible while close operation.
But this bug is too serious to use this functionality so I've reverted to ol'good "desktop=" which works flawlessly.

Is it RAC DB?

Thanks Marco. With that NLS_LANG everything is working fine.

It should be available by the end of this month.