you may b using the wrong link in the dropDown for embed codes on imgbb.com ??
for example, on upload of random photo to imgbb domain, your given a list of ways ( embed links ) to request the photo selection from list → encoding of the response
AND perhaps you supply the incorrect choice when you try to init the bubble api connector
So , just try the other links each time retrying the connector.init action … one of them should result in a good init.
its kinda like what i did below , in order to determin if the browser would render the links…
NOTE your error code on the init is saying it does not like the byte format it was given in the body ( photo field of the connector form )
** HTML full linked** is the drop down selection value that i used above to get the browser to render the tag…