Hey Sascha,
I should really start by saying that we have lots of users who use Simple:Press along side WooCommerce with no problem at all, and so far have not had any other reports of what sounds like a jquery conflict due to WooCommerce.
Just for peace of mind, ‘javascript:void(null)’ is not an error or a problem of any kind – see This Page for reference.
So as to the problem itself, is everything up to date i.e SP and SP plugins, WP and WP plugins, WooCommerce and any of it’s additions?
What has changed that you know of? You’ve said ‘working like in past’ so I’m assuming it worked fine meaning something must have changed. Have you added any WooCommerce extensions or third party plugins that run along with WooCommerce that could be to blame?
Make sure you are not caching the forum page too, as Simple:Press is dynamically loaded on one page it can’t be cached. Not sure if WooCommerce offer any kind of caching but it’s worth me mentioning.
And lastly, are you able to open the browsers console when viewing a page with the problem to see if any errors / conflicts are showing?