Aah, thanks for pointing me at that log folder, I've only been running this site for two weeks so I haven't found everything yet.
That file has a bunch of stuff in it, but it really doesn't seem to be helpful:
Thu, 09 Apr 2020 15:25:51 +0000
OutOfRangeException: (0)
#0 /var/www/html/system/Theme/Theme.php(373): IPS\Patterns\_ActiveRecord::load()
#1 /var/www/html/system/Dispatcher/Standard.php(54): IPS\_Theme::i()
#2 /var/www/html/system/Dispatcher/Front.php(673): IPS\Dispatcher\_Standard::baseCss()
#3 /var/www/html/system/Dispatcher/Front.php(77): IPS\Dispatcher\_Front::baseCss()
#4 /var/www/html/system/Dispatcher/Dispatcher.php(109): IPS\Dispatcher\_Front->init()
#5 /var/www/html/index.php(13): IPS\_Dispatcher::i()
#6 {main}
#0 /var/www/html/init.php(898): IPS\_Log::log()
#1 [internal function]: IPS\IPS::exceptionHandler()
#2 {main}
-------------
Thu, 09 Apr 2020 15:25:52 +0000
OutOfRangeException: (0)
#0 /var/www/html/system/Member/Member.php(747): IPS\Patterns\_ActiveRecord::load()
#1 /var/www/html/system/Patterns/ActiveRecord.php(335): IPS\_Member->get_group()
#2 /var/www/html/system/Session/Front.php(187): IPS\Patterns\_ActiveRecord->__get()
#3 [internal function]: IPS\Session\_Front->read()
#4 /var/www/html/system/Session/Session.php(91): session_start()
#5 /var/www/html/system/Theme/Theme.php(389): IPS\_Session::i()
#6 /var/www/html/system/Theme/Theme.php(289): IPS\_Theme::currentThemeId()
#7 /var/www/html/system/Dispatcher/Standard.php(54): IPS\_Theme::i()
#8 /var/www/html/system/Dispatcher/Front.php(673): IPS\Dispatcher\_Standard::baseCss()
#9 /var/www/html/system/Dispatcher/Front.php(77): IPS\Dispatcher\_Front::baseCss()
#10 /var/www/html/system/Dispatcher/Dispatcher.php(109): IPS\Dispatcher\_Front->init()
#11 /var/www/html/index.php(13): IPS\_Dispatcher::i()
#12 {main}
#0 /var/www/html/init.php(898): IPS\_Log::log()
#1 [internal function]: IPS\IPS::exceptionHandler()
#2 {main}
If it helps, most of my filesystem is off the webserver, on S3 including my JS/CSS caches. But I'm not sure why things are breaking - it's the same webserver running the same fileserver but if I move the database using that command and point to the new server the whole things falls in a heap. If I switch back, everything works again.
Actually, scratch that, all the Storage Settings options are set to use S3 instead of the local filesystem.