Yes, basically all non MG pages. What I've just tried is switching to php7.4 from FPM I then tried turning on MG. New errors appeared (below)
It did give me a hint though. So I'm running Resycn media. But I've unselected all mirred galleries. If this does not fix it I'll post a report.
#0 src/XF/Mvc/Entity/Entity.php(115): XF\Mvc\Entity\Entity->get('XfmgMirrorMedia')
#1 internal_data/code_cache/templates/l1/s15/public/xfmg_attachment_mirror_macros.php(12): XF\Mvc\Entity\Entity->offsetGet('XfmgMirrorMedia')
#2 src/XF/Template/Templater.php(799): XF\Template\Templater->{closure}(Object(SV\StandardLib\XF\Template\Templater), Array, NULL)
#3 src/XF/Template/Templater.php(836): XF\Template\Templater->callMacro('xfmg_attachment...', 'lightbox_captio...', Array, Array)
#4 src/XF/Template/Templater.php(2398): XF\Template\Templater->renderMacro(NULL, 'xfmg_attachment...', Array)
#5 [internal function]: XF\Template\Templater->fnCallMacro(Object(SV\StandardLib\XF\Template\Templater), false, NULL, 'xfmg_attachment...')
#6 src/XF/Template/Templater.php(1103): call_user_func_array(Array, Array)
#7 internal_data/code_cache/templates/l1/s15/public/attachment_macros.php(24): XF\Template\Templater->func('call_macro', Array, false)
#8 src/XF/Template/Templater.php(799): XF\Template\Templater->{closure}(Object(SV\StandardLib\XF\Template\Templater), Array, NULL)
#9 internal_data/code_cache/templates/l1/s15/public/message_macros.php(312): XF\Template\Templater->callMacro('attachment_macr...', 'attachment_list...', Array, Array)
#10 src/XF/Template/Templater.php(799): XF\Template\Templater->{closure}(Object(SV\StandardLib\XF\Template\Templater), Array, NULL)
#11 internal_data/code_cache/templates/l1/s15/public/post_macros.php(462): XF\Template\Templater->callMacro('message_macros', 'attachments', Array, Array)
#12 src/XF/Template/Templater.php(799): XF\Template\Templater->{closure}(Object(SV\StandardLib\XF\Template\Templater), Array, NULL)
#13 internal_data/code_cache/templates/l1/s15/public/post_macros.php(60): XF\Template\Templater->callMacro('post_macros', 'post_user_conte...', Array, Array)
#14 src/XF/Template/Templater.php(962): XF\Template\Templater->{closure}(Object(SV\StandardLib\XF\Template\Templater), Array, Object(XF\Template\ExtensionSet))
#15 src/XF/Template/Templater.php(898): XF\Template\Templater->renderExtensionInternal('user_content', Array, Array, Object(XF\Template\ExtensionSet))
#16 internal_data/code_cache/templates/l1/s15/public/post_macros.php(94): XF\Template\Templater->renderExtension('user_content', Array, Object(XF\Template\ExtensionSet))
#17 src/XF/Template/Templater.php(962): XF\Template\Templater->{closure}(Object(SV\StandardLib\XF\Template\Templater), Array, Object(XF\Template\ExtensionSet))
#18 src/XF/Template/Templater.php(898): XF\Template\Templater->renderExtensionInternal('content', Array, Array, Object(XF\Template\ExtensionSet))
#19 internal_data/code_cache/templates/l1/s15/public/post_macros.php(123): XF\Template\Templater->renderExtension('content', Array, Object(XF\Template\ExtensionSet))
#20 src/XF/Template/Templater.php(962): XF\Template\Templater->{closure}(Object(SV\StandardLib\XF\Template\Templater), Array, Object(XF\Template\ExtensionSet))
#21 src/XF/Template/Templater.php(898): XF\Template\Templater->renderExtensionInternal('main_cell_inner', Array, Array, Object(XF\Template\ExtensionSet))
#22 internal_data/code_cache/templates/l1/s15/public/post_macros.php(137): XF\Template\Templater->renderExtension('main_cell_inner', Array, Object(XF\Template\ExtensionSet))
#23 src/XF/Template/Templater.php(962): XF\Template\Templater->{closure}(Object(SV\StandardLib\XF\Template\Templater), Array, Object(XF\Template\ExtensionSet))
#24 src/XF/Template/Templater.php(898): XF\Template\Templater->renderExtensionInternal('main_cell', Array, Array, Object(XF\Template\ExtensionSet))
#25 internal_data/code_cache/templates/l1/s15/public/post_macros.php(149): XF\Template\Templater->renderExtension('main_cell', Array, Object(XF\Template\ExtensionSet))
#26 src/XF/Template/Templater.php(962): XF\Template\Templater->{closure}(Object(SV\StandardLib\XF\Template\Templater), Array, Object(XF\Template\ExtensionSet))
#27 src/XF/Template/Templater.php(898): XF\Template\Templater->renderExtensionInternal('full_body', Array, Array, Object(XF\Template\ExtensionSet))
#28 internal_data/code_cache/templates/l1/s15/public/post_macros.php(179): XF\Template\Templater->renderExtension('full_body', Array, Object(XF\Template\ExtensionSet))
#29 src/XF/Template/Templater.php(799): XF\Template\Templater->{closure}(Object(SV\StandardLib\XF\Template\Templater), Array, Object(XF\Template\ExtensionSet))
#30 internal_data/code_cache/templates/l1/s15/public/thread_view.php(809): XF\Template\Templater->callMacro('post_macros', 'post', Array, Array)
#31 src/XF/Template/Templater.php(1626): XF\Template\Templater->{closure}(Object(SV\StandardLib\XF\Template\Templater), Array, Object(XF\Template\ExtensionSet))
#32 src/XF/Template/Template.php(24): XF\Template\Templater->renderTemplate('thread_view', Array)
#33 src/XF/Mvc/Renderer/Html.php(48): XF\Template\Template->render()
#34 src/XF/Mvc/Dispatcher.php(458): XF\Mvc\Renderer\Html->renderView('XF:Thread\\View', 'public:thread_v...', Array)
#35 src/XF/Mvc/Dispatcher.php(440): XF\Mvc\Dispatcher->renderView(Object(XF\Mvc\Renderer\Html), Object(XF\Mvc\Reply\View))
#36 src/XF/Mvc/Dispatcher.php(400): XF\Mvc\Dispatcher->renderReply(Object(XF\Mvc\Renderer\Html), Object(XF\Mvc\Reply\View))
#37 src/XF/Mvc/Dispatcher.php(58): XF\Mvc\Dispatcher->render(Object(XF\Mvc\Reply\View), 'html')
#38 src/XF/App.php(2326): XF\Mvc\Dispatcher->run()
#39 src/XF.php(488): XF\App->run()
#40 index.php(20): XF::runApp('XF\\Pub\\App')
#41 {main}