While trying to load a mp4 video (HTML5 was chosen), I got the message The server returned an invalid JSON response.

JCE JCE Pro 2.9.55
Joomla 5.0.1

Strange: I did the the same on 4 other sites without any problem.

[quotePost id="115419"]Does this happen with any mp4 file or just this file?

What about other file types, eg: images?[/quotePost]

Only with this file. As I wrote before: the same file was successfully added on other websites also running the same software..

[quotePost id="115428"][quotePost id="115419"]Does this happen with any mp4 file or just this file?

What about other file types, eg: images?[/quotePost]

Only with this file. As I wrote before: the same file was successfully added on other websites also running the same software..[/quotePost]

[quotePost id="115429"]On this site, is Validate Mimetype enabled in Editor Profiles -> Editor Parameters -> Filesystem?[/quotePost]
Yes

[quotePost id="115432"]If you set it to No, do you still get the error?[/quotePost]

That solved the problem. THX !