I just wanted to say that I’m having the same issue mentioned by multiple others here. I’ve looked at this thread and all the similar ones, spent hours troubleshooting, no solution. Here is what I’ve verified.
1. It is absolutely a result of the 5.6 update.
2. I have disabled ALL plugins except siteorigin, doesn’t help.
3. Restored from backup multiple times now trying different things. Tried disabling all security software before update in case that was blocking something. (including wordfence, which another user reported as causing problems)
4. Tried “classic editor” plugin, no difference. The page list says “siteorigin page builder” as usual, but when you edit the page, you only get the “visual” and “text” tabs. Normally, you see those tabs flash momentarily then it automatically goes to the “page builder” tab and hides everything else until you click “revert editor”
5. Tried clearing site cookies locally. Did a master flush on siteground cache.
6. Tried a different browser with a full cache/history/data clear. (Firefox)
7. Tried opening on my phone since another user said that worked. Nope.
The most curious part of this is that after everything initially broke, I created a staging copy to experiment with fixes. Before I changed anything at all, the staging copy worked perfectly! However, if I push the working staging copy to the live site, it completes successfully but site origin is back to being broken same as before. I have no idea what could explain this.
For now, I’ve restored the backup again and will not update to WP 5.6 until this can be figured out.