After updating from TinyMCE 4 to TinyMCE 5, some migration steps are necessary to display content from files (e.g., images) in TinyMCE.
If you use some old [ismediaobject] links, such as
<p> <img src="https://./?[ismediaobject]isfile://inSPIRED-Site/inTRONICS_Business-smb-responsive/inSPIRED-inTRONICS_Business-smb-responsive/en_US/documentation/quick_order_link.png|/INTERSHOP/static/WFS/inSPIRED-Site/inTRONICS_Business-smb-responsive/inSPIRED-inTRONICS_Business-smb-responsive/en_US/documentation/quick_order_link.png[/ismediaobject]" alt="Quick Order link" width="583" height="38" /> </p>
you need to replace them with a valid URL, such as
<p> <img src="/INTERSHOP/static/WFS/inSPIRED-Site/inTRONICS_Business-smb-responsive/inSPIRED-inTRONICS_Business-smb-responsive/en_US/documentation/quick_order_link.png" alt="Quick Order link" width="583" height="38" /> </p>
Afterwards, the content is visible in TinyMCE.