When Qt runs it looks for its plugins folder relative to where the application started. Some Python environments (including Anaconda) install the Qt plugins relative to the python executable so that when running Python, Qt can find its plugins relative to the python.exe application.
When using PyXLL, the application is not python.exe but EXCEL.EXE. This means that Qt will not be able to find its plugins by looking in a folder relative to the EXCEL.EXE application.
To fix this you should set the QT_PLUGIN_PATH environment variable to the location of the Qt plugins folder. There is no need to set this globally, as that might conflict with other Qt applications. Instead, you should set it in your pyxll.cfg file as follows - but replace the actual path with the plugins folder in your own Python installation.
QT_PLUGIN_PATH = C:\Users\xxx\Anaconda\envs\pyxll\Library\plugins
For more information about how Qt locates its plugins folder and other options for setting the plugins path, see https://doc.qt.io/qt-5/deployment-plugins.html from the Qt documentation.
Setting environment variables in the pyxll.cfg config file is explained in more detail in the PyXLL docs here https://www.pyxll.com/docs/userguide/config.html#environment-variables.