ReadOnlyStream::stream_open" call failed

Hi there! I try to upload file (JPG) on frontend Form. Form is displayed but when clicking Send button, this Error message is thrown:

copy(plugins://fetch-from-fm/images/10_Xenix_Filmdistr.jpg): failed to open stream: “\RocketTheme\Toolbox\StreamWrapper\ReadOnlyStream::stream_open” call failed

What could the be. Have not found anything similar in this forum nor on the net.
Thanks for getting back to me.

Cheers, J