More like 48+ hours across 3 days, nominally lunchtime Monday to lunchtime Wednesday.
That is a wholly unacceptable level of service, especially when it's is flagged as 500 Error for Server Unavailability.
There's plenty of help pages on the internet for 500 Errors, perhaps our hosting company should read some of them!
But to answer your question, I suspect it's 2 fold. Stop using a creaking out of date software and get a better hosting & support provider.
What I knows about PHPBB software could probably be written on the proverbial postage stamp, but I've done a little bit of digging around, and as far as I can ascertain, this site is is possibly missing a few years worth of updates compared to the latest PHPBB version. Only someone like our Webmaster Mike would know the details.
It may be that the log details I looked at simply haven't been updated when the updates were carried out; but even that to me is sloppy workmanship.