Website with HTML code since WP update or installation of Plugin Yast SEO?

Hello,
my website(s) show HTML code now (customer view) and the Enfold editor isn’t shown/usable. The is only a lot of HTML code shown.
I’m not sure if the problem is the WP update to 5.X or installation of plugin Yast SEO (in meantime deinstalled, but problem stays).
In the meantime I switched to the classic editor again, but had no effect.
I’m not experienced with WP, used a homepage builder before, but switched to WP Hosting and Enfold Theme for more possibilities.
Here a short description of my system landscape:
# 1&1 WordPress Hosting
# Multisite:
4 Websites with different Designs
# Plugins/Themes:
– WP MultiPurposeTheme “Enfold V.4.5” mit Avia Layout Builder (instead of the standard WP editor)
– DuplicatePost
– BackWPup
– a3 Lazy Load

Example of failure (shown to endusers on website):
RALF WERKLE<br /> – Der VäterErMuTiger -<br /><br /><br /><br /><br />

Working on the websites, the Enfold Editor isn’t shown any more, but HTML-Coding:
<p>[av_headline_rotator before_rotating='RALF WERKLE' after_rotating='' multiline='aviaTBmultiline' interval='7' animation='fade' margin='' margin_sync='true' tag='h1' size='3vw' align='center' custom_title='' av-medium-font-size-title='' av-small-font-size-title='' av-mini-font-size-title='' av_uid='av-jo4k6qs8' admin_preview_bg='']<br />[av_rotator_item title=' - Der VäterErMuTiger -' link='' linktarget='' custom_title='' av_uid='av-3kl8xf']<br />[av_rotator_item title=' - Väter- und MännerMentor -' link='' linktarget='' custom_title='' av_uid='av-3fmxy3']<br />[av_rotator_item title='- Vater werden ist nicht schwer, Vater sein dagegen sehr -' link='' linktarget='' custom_title='' av_uid='av-38offf']<br />[av_rotator_item title='- Ziel ist es, das unnötige Leiden in den Familien zu beenden. Durch Befähigung und Bewusstwerdung von Vätern, Müttern, Eltern! -' link='' linktarget='' custom_title='' av_uid='av-b1amb']<br />[av_rotator_item title='' link='' linktarget='no' custom_title='' av_uid='av-2wx9mr']<br />[/av_headline_rotator][av_fullscreen size='extra_large' image_attachment='' animation='slide' conditional_play='' autoplay='true' interval='5' control_layout='av-control-default' scroll_down='aviaTBscroll_down' av_uid='av-jo4skv92']<br />[av_fullscreen_slide id='422' av_uid='av-2ovsc3'][/av_fullscreen_slide]<br />[av_fullscreen_slide id='419' av_uid='av-2ms5cb'][/av_fullscreen_slide]<br />[av_fullscreen_slide id='421' av_uid='av-2dr8kz'][/av_fullscreen_slide]<br />[av_fullscreen_slide id='418' av_uid='av-28jqkz'][/av_fullscreen_slide]<br />[av_fullscreen_slide id='406' av_uid='av-207vtv'][/av_fullscreen_slide]<br />[av_fullscreen_slide id=’407′
…
…

I’m not sure what to do. I thought about updating the enfold theme, but did the mistake not to use the enfold child theme.
Therefore I expect the loss of my developments with an update to higher version.
What do you recommend? Can s.b. assist my with transferring the developments to the enfold child theme before upgrading?
What is the best way and the next steps?

Thanks in advance.
BR, Ralf.