Hi - I've had this error start happening recently, and it seems linked to Sucuri firewall. I used to be able to upload images via JCE with no problem, but now it seems to have shut me out. I'm not sure what's changed, to be honest. 

I can get around this by going on a whitelisted static IP, but I have to change to that every time I want to upload something. 

Just wondering - have you come up against this problem with firewalls before, and is there a way to avoid this?

I am encountering the same error as mentioned above.
Has this issue been resolved?

Current status of my site:
・The following error occurs when uploading an image, preventing the upload.

Joomla 6.1.2
JCE Pro 2.9.99.9
Image details: .jpg, 10KB

I have checked all MIME settings in both Joomla and JCE.
The server's WAF is also turned "off."
I contacted the server provider's support team, and they stated that no changes have been made recently.

For me, this is only a very intermittent issue now, works about 90% of the time. 

For those times it doesn't, going on a whitelisted static IP definitely solves the issue 100%. Not an ideal fix for everyone, but it is a way around it.  

Thank you, gavhanly

I'd like to try your method, but how do you achieve "whitelisting" as you mentioned?

Thank you

First work out what your IP is, then add that to the Sucuri whitelist. If you don't have access to that, ask your host to do it for you. If your IP changes all the time (like mine does), you might need to get a static IP from something like NordVPN and get that whitelisted. That worked for me. 

It is taking a long time to resolve this issue.
It appears to be quite a difficult problem.
Is there any workaround, such as deleting and reinstalling JCE?