There are many issues. The problem is, no one seems to know, with certainty, what they are. There have been 'some' fixes and that's obvious. But, someone supposedly who works in the industry volunteered some time from he and his 'team' a month or two back and the only thing we've heard has been found on another site. (We won't get into the back and forth over whether anyone is actually working on it. One either has faith or they don't and it's either misplaced or it's not.)
Which brings me to the point. There are many, many, many threads on .org and .net regarding what is 'wrong.' A lot of theories. A lot of expertizing. A lot of 'offers.' Ultimately, we're only marginally better off than we were. The site works like Gang Busters (got that listeners...

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https://en.wikipedia.org/wiki/Gang_Busters ) for short periods, sometimes several times in a given day. Other times, it can be unreachable for many of us over 24 - 36 hrs. and, for some, even longer.
The one thing we're reasonably sure of is that it's not as 'simple' as many are trying to claim it is. Kestryll warned us, for years, this was likely to happen with an upgrade. He was hanging on to iTrader for those trading; but, iTrader only worked with the version of vBulletin the site was based on and not the newer iterations. Short version... The hosting site went out of business, the one it was moved to wanted too much money, the one it's currently on forced an upgrade. The site history (all the threads, etc.) appears to have been preserved. Yet, at its base, it's a matter of making those threads work with new software several iterations more current than what has been preserved.
I hope that helps.