vnStat vnStat web UI stopped working

[Oracle]

The web UI page for dn-vnStat doesn't load anymore.
Not only does it not load but it causes httpd to hit 100% CPU use and block the entire router web UI.
Router model AC86U on Merlin 386.7.

This problem appeared right after I upgraded Diversion from 4.2. to 4.3. I don't see the connection between the 2 (maybe there isn't any) but one happened right after the other.

 

[dave14305]

Check the files in /www/ext/dn-vnstat/. Anything really large or old looking? Never used this, but I would start by assuming these data files are part of the problem.

 

[Oracle]

This will be a challenge to diagnose.

So if I reboot the router and go to the vnStat web UI page (Addons -> dn-vnstat), the page loads.
If I then go and open a Diversion page (LAN -> Diversion) and then come back to the dn-vnstat page, the whole router web UI freezes. I can't say how consistent this is - already had to reboot 3 times via PuTTY because the web UI stopped responding.

vnStat works by generating usage images every 5 min. Something seems to interfere with the images creation, because they have occasionally started to come out incomplete.

 

[Oracle]

This problem persists, although less severe (or I got used to it and avoid clicking too much around the web UI).
Data appears to be fine but every now and then the page would freeze or images load partially.
Let's say - around half the time. I.e., images are broken, after 5 min they are ok, next refresh - broken again. By "broken" I mean incomplete, like on the attachment above.