I'm sorry Jozef, I've tested it last week and I completly forgot to say that the new version of slf4J solves the problem. Upgrading SLF4J is good workaround for me.
Do you know how to force passivation programmatically (or using the JBoss UI)
Not sure how to "force" it. What I did in a test is that I try to serialize both the bean and its creational context https://github.com/weld/core/commit/41d85329788772bb871bf981c36a9538cc96daed#diff-5
Ok great that is good to know. Your test is much more eleguant that mine as I used the client side approach and I was therefore not able to catch the exception.
Regarding forcing passivation, the other solution that I found was to limit the number of simultaneous sessions to 1 and to perform 2 requests with two different clients.
Many thanks again for your help