My webhoster uses open_basedir. I am not familar with it. There are no /usr/bin in configured paths. But it seems not to stop PHP from start programs from /usr/bin there binaries like imagemagic and netpbm needet by appropriate plugins.
From 2.2.6 to 2.3.0 they are some changes in <Plugin>/classes/<Plugin>ToolkitHelper.
This Changes seems to make sense on the first look but the original comment and testing on my webhosting show what who make this changes could missunderstood the functionality of open_basedir.
It realy helps for me not to stop if function "isRestrictedByOpenBaseDir" gives true. It seems, that you just cannot rely on something like is_dir if open_basedir is used and it was the only intention on 2.2.6 code.
What I dit is revert the change respectively changes back c&p changed code.
The patched plugins are imagemagic and netpbm.
Similar error occures with jpegtran and maybe other plugins that relly on a valid path to binary tools.
In case of jpegtrans I could not get working patch for me. Don't know why. (The binary is installed on webhoster and in the right version). Someone may have a look on jpegtran and maybe others (e.g. ffmpeg) that I did not test.