Hi unpolbo,
This warning should not affect the final rendering, it happens sometimes, and only when reading the rendered image the first time.
To avoid this message, you can uncheck the 'Enable Extra Warnings' in Octane global preferences, tab 'Trouble Shooting'.
Alternatively, you could try to set the Render width and height in Kernel settings same as the Output size in Common tab, and uncheck the 'Use production resolution'.
I hope this will fix your problem.
A better fix will be added in the next release, 4.40
Thanks