500 Error on reply or post

I'm getting a server 500 error on every new message and every reply I post, though they seem to be posting OK.

I'm just getting a white page.

I'm also not seeing new posts or unread replies at the top, even though there are some.

yes, seems the process, which runs after posting something is broken somehow, since yesterday.
I get also allways a blank screen after posting.

We performed a PHP upgrade yesterday and the Solr extension got lost in the shuffle. It should be re-enabled now and the 500 errors should be gone.