Do you have a sense of how many blocks are in your project? Really large projects sometimes produce images that are too large and result in the browser rejecting the image since it's too big. If that's the case, you can try to go block by block downloading the images.
The image shown above shows 164 blocks only.....
In that case, I do think the next thing to check is whether you're blocked popups from App Inventor. Something else that can help us further is if you open Firefox's developer tools (Ctrl+Shift+I) and tell us if after you do the "Download Blocks as PNG" command whether any errors appear.
they are not blocked
I'll do that
If it helps, just tested a blocks download on my Xubuntu 20.04 Linux box with Firefox. No issues here.
This error appears when i click the button:
"Uncaught DOMException: CSSStyleSheet.cssRules getter: Not allowed to access cross-origin stylesheet line 9 > injectedScript:36"
more details:
svgAsDataUri http://ai2.appinventor.mit.edu/ line 9 > injectedScript:36
saveSvgAsPng http://ai2.appinventor.mit.edu/ line 9 > injectedScript:36
onclickExportBlocks http://ai2.appinventor.mit.edu/ line 9 > injectedScript:36
callback http://ai2.appinventor.mit.edu/ line 9 > injectedScript:31
fireListeners http://ai2.appinventor.mit.edu/ line 9 > injectedScript:14
dispatchEventInternal_ http://ai2.appinventor.mit.edu/ line 9 > injectedScript:14
dispatchEvent http://ai2.appinventor.mit.edu/ line 9 > injectedScript:14
performActionInternal http://ai2.appinventor.mit.edu/ line 9 > injectedScript:16
handleMouseUp http://ai2.appinventor.mit.edu/ line 9 > injectedScript:16
base http://ai2.appinventor.mit.edu/ line 9 > injectedScript:2
handleMouseUp http://ai2.appinventor.mit.edu/ line 9 > injectedScript:16
handleChildMouseEvents http://ai2.appinventor.mit.edu/ line 9 > injectedScript:16
fireListener http://ai2.appinventor.mit.edu/ line 9 > injectedScript:11
handleBrowserEvent_ http://ai2.appinventor.mit.edu/ line 9 > injectedScript:11
e http://ai2.appinventor.mit.edu/ line 9 > injectedScript:11
(Async: EventListener.handleEvent)
listen_ http://ai2.appinventor.mit.edu/ line 9 > injectedScript:11
listen http://ai2.appinventor.mit.edu/ line 9 > injectedScript:11
listen_ http://ai2.appinventor.mit.edu/ line 9 > injectedScript:16
listen http://ai2.appinventor.mit.edu/ line 9 > injectedScript:16
enterDocument http://ai2.appinventor.mit.edu/ line 9 > injectedScript:16
render_ http://ai2.appinventor.mit.edu/ line 9 > injectedScript:16
render http://ai2.appinventor.mit.edu/ line 9 > injectedScript:16
show http://ai2.appinventor.mit.edu/ line 9 > injectedScript:16
showContextMenu_ http://ai2.appinventor.mit.edu/ line 9 > injectedScript:16
onMouseDown_ http://ai2.appinventor.mit.edu/ line 9 > injectedScript:14
h http://ai2.appinventor.mit.edu/ line 9 > injectedScript:25
n http://ai2.appinventor.mit.edu/ line 9 > injectedScript:21
What kind of script blocking extensions do you have?
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@Anke Thanks for that. While I can't replicate the issue on my end I think I can make a potential patch.
@GuiM_Haes: Please try to upload your project to http://png-export.ai2-ewpatton-temp.appspot.com/ and see if you can export the blocks using this version.
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My problem is solved - Thanks @Anke. multiple persons talked about ghostery as a problem. I tried pausing ghostery and it worked.
Thanks everyone!
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I already tried this because i knew of that website. @anke's solution worked, just disabling ghostery.
I'm not sure what you mean by "I already tried this because I knew of that website". I literally wrote that code a few minutes ago so it couldn't have existed before my post.
i already tried the website mempot.ai2-ewpatton-temp.appspot.com and it contains the same projects as in png-export.ai2-ewpatton-temp.appspot.com.
Well, this question was first posed by AGB and later by Evan, and now you tell us you do have a pop-up blocker and switching it off solves the issue.........
its not a pop-up blocker, its ad and trackers blocker
....some of which are pop-ups GuiM! 
Note also from the same report that the Dark Mode and Dark Reader extensions also inject web page changes and therefore might also have a detrimental effect on App Inventor, which is not a simple web page but a web-based program.
If you need to retain ghostery, you could dedicate a separate browser to APP Inventor, thus avoiding issues all around. If that were Chrome, you wouldn't need to worry about Google breaching your privacy because you would not be using it to browse.
i can disable ghostery for app inventor or pause it while i go to app inventor. It is the best for me
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The fix I submitted to the rest of the team for review should resolve the negative interaction with extension-based stylesheets, so hopefully this won't be an issue in future versions of App Inventor.
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