Hi, thank you very much for this great program. Last night I uninstalled amdgpu-pro then upgraded from Ubuntu 16.04 to 18.04. I then install amdgpu-pro 18.40-697810 with the --opencl=legacy command line option.
The current version of PhoenixMiner 4.2c supports AMD Linux drivers up to 18.40-676022 (your driver is the next one and is not supported by the released version). You need to uninstall amdgpu-pro and then download and install the 18.40-676022 drivers. We will add support for all newer drivers in the next release of PhoenixMiner but obviously nothing stops AMD from releasing newer drivers after that and therefore we recommend to avoid upgrading to drivers that are newer than the version of the PhoenixMiner you are using.
Thank you sir! 18.40-676022 solved my issue.