It first happened to the user who was trying to add a wikilink to a large page, https://en.wikipedia.org/wiki/List_of_Criterion_Collection_DVD_and_Blu-ray_releases; he got the error while in VE so he made that edit with the classic editor later.
He then copied the contents of the page to a sandbox - https://en.wikipedia.org/wiki/User:Marc_Kupper/sandbox2 - for further testing and these are his findings:
<< It took 74 seconds to load the page in VE, I made the change, save-page, give a reason (wikilink - testing VE), save-page again. After 1:45 (105 seconds) I got "Error: Unknown error" in the lower part of VE's edit-summary box. The save-page button is grayed out.
I don't think the actual edit I did matters. If someone wants to reproduce using my sandbox then the edit was in the last paragraph of the lead. Where it says "...Essential Art House, Eclipse, and Merchant Ivory Collection..." I changed "Eclipse" to [[Eclipse (DVD)|Eclipse]] by double-clicking on Eclipse to select, control-K to bring up the insert/edit link box, appending " (d" to the link-to value, click "Eclipse (DVD)" in the list of offerings, and then press <enter> a couple of times to close out the box. I suspect you can make any change you want. --Marc Kupper|talk 22:06, 19 August 2013 (UTC)
After documenting how I did the edit I tried the save again and again got the error. It happened 1:44 (104 seconds) after clicking save. I suspect the 105 seconds above is simply because I was a bit slow reacting as this time I was waiting for the error right around the 1:45 mark. --Marc Kupper|talk 22:06, 19 August 2013 (UTC)
I then did the following tests
I replaced all non-alphabetic characters with the letter "x" and can edit the page with VE. It appears the issue is with something in the page content, possibly combined with the page size.
I removed all 'ref' tags. Saving from VE still fails after 1:44
I removed most of the templates transcluded on the page. Saving with VE worked and took 1:17. However, I had intended to just add "xxx" in the middle of a word. VE also stripped a table in another part of the article of wikitext. (https://en.wikipedia.org/w/index.php?title=User%3AMarc_Kupper%2Fsandbox2&diff=569295763&oldid=569295507 ) --Marc Kupper|talk 23:16, 19 August 2013 (UTC)
I don't have time to debug this further at present - particularly as we now have two VE issues. The next step is to revert back to 22:48, 19 August 2013 and to then see if I can narrow it down from there. --Marc Kupper|talk 23:11, 19 August 2013 (UTC) >>
I tested on the first version of the sandbox as well, it took me almost 75 secs as well to load the page, I wikilinked a word and then hit Save - it took me 2min 15sec to get the error message while actually the edit was saved. Thanks.