I did a load of Collatz WUs yesterday, but nothing has been credited.
I was surprised to see no points in yesterday's total, and there's nothing carried forward to today...
Has there been an issue gathering their results?

Same here.

But interestingly, the 'thesonntags' site result on Google contains the warning "This site may be hacked."

Indeed.
Looks like the site certificate may have expired.

"NET::ERR_CERT_DATE_INVALID
This server could not prove that it is boinc.thesonntags.com; its security certificate expired 3 days ago."

Still got no credit pending.......

The Collatz web site SECURITY (SSL) CERTIFICATE HAS EXPIRED.

When Collatz gets a UPDATED VALID CERTIFICATE the various stats sites will again be able to download and process the stats from there.

Slicker posted on the Collatz 'News' board on the 28th:

"After finally getting the site cleared by Google's security scan, I was able to apply for and install a new SSL certificate. Thanks for your patience."

However, as of a minute ago, Google is still flagging the site, making it still necessary to add a exception if you want to log in there.

Other users have subsequently pointed this out in replies to Slicker's post, but without any acknowledgment, or, apparently, action from Slicker.

So I guess we just wait...

Same. No points last 4 days

WAIT YOU WILL!
That post/message was on 28 Oct 2015, 13:29:26 UTC ----- OVER 1 YEAR AGO

Again no updated stats from Collatz. I just wonder why Free-DC has correct stats (at least for the last two days).

Slicker's post was a year ago...

I think Slicker goes hunting around this time every year. If there is a problem at Collatz, you may not see a resolution for a week or so.

...and now the project is offline altogether.

Well, project is back up and the SSL certificate has been replaced.
Still seeing no credit on here though... approx 1.37 million short.

11/8/2016 12:00:04 AM | Collatz Conjecture | update requested by user
11/8/2016 12:00:09 AM | Collatz Conjecture | Fetching scheduler list
11/8/2016 12:00:12 AM | | Project communication failed: attempting access to reference site
11/8/2016 12:00:14 AM | | Internet access OK - project servers may be temporarily down.

Also no new work or reporting

The project doesn't export its stats since November 8th and therefor BOINCstats can't show the latest credit.

It not just not report stats. It not working at all !

Collatz admin guy is saying that isn't the case:

[Quote]
That is not correct.
drwxrwx--x 2 boincadm boincadm 4096 Nov 9 06:46 stats
drwxrwx--x 2 boincadm boincadm 4096 Nov 7 02:46 stats_2016_11_7_6_46_48
drwxrwx--x 2 boincadm boincadm 4096 Nov 8 06:46 stats_2016_11_8_10_46_47
drwxrwx--x 2 boincadm boincadm 4096 Nov 8 10:46 stats_2016_11_8_14_46_47
drwxrwx--x 2 boincadm boincadm 4096 Nov 8 14:46 stats_2016_11_8_18_46_46
drwxrwx--x 2 boincadm boincadm 4096 Nov 8 18:46 stats_2016_11_8_22_46_45
drwxrwx--x 2 boincadm boincadm 4096 Nov 7 22:46 stats_2016_11_8_2_46_46
drwxrwx--x 2 boincadm boincadm 4096 Nov 8 02:46 stats_2016_11_8_6_46_47
drwxrwx--x 2 boincadm boincadm 4096 Nov 8 22:46 stats_2016_11_9_2_46_47
drwxrwx--x 2 boincadm boincadm 4096 Nov 9 02:46 stats_2016_11_9_6_46_47


drwxrwx--x 2 boincadm boincadm 4096 Nov 9 06:46 .
drwxr-xr-x 19 boincadm boincadm 32768 Nov 9 06:46 ..
-rw-r--r-- 1 boincadm boincadm 701 Nov 9 06:46 db_dump.xml
-rw-r--r-- 1 boincadm boincadm 56780605 Nov 9 06:46 host.gz
-rw-r--r-- 1 boincadm boincadm 493 Nov 9 06:46 tables.xml
-rw-r--r-- 1 boincadm boincadm 275520 Nov 9 06:46 team.gz
-rw-r--r-- 1 boincadm boincadm 4174989 Nov 9 06:45 user.gz
[End Quote]

Hurrah! Seems to be working again!

19,820,916.54
Comprising 17,398,094.32 from the daily update
+ 2,422,822.22 since then

Yes working, but no stats !

Now stats also works.