Let's perform some troubleshooting steps to get this fixed and get you back to working order.
There are times that the browser is full of frequently-accessed page resources, thus causing websites, like QuickBooks Online (QBO), to act weirdly. Try logging into your QBO account using a different or a private browser.
To use a private browser, here's how:
Google Chrome: Ctrl + Shift + N
Internet Explorer: Ctrl + Shift + P
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
Once logged in, go back and try to check if your uploaded logo shows on sales forms.
However, if your uploaded logo displaying "X" on sales forms, there is an on-ongoing issue where users experiencing the same thing. Rest assured that our product engineers are diligently working on a fix.
To ensure you'll get an update about the resolution status, I recommend reaching out to our Technical Support Team. They have to collect personal data to add your company and product to the list of affected users and provide this investigation number for easy tracking: INV-38042.
To reach them, click Help (question mark icon) at the top right, then hit Contact us.
Thank you for your patience as we work for a fix. Please know that you can always get back to this post if you have any other questions.