The fragment shader in:

https://cgit.kde.org/plasma-workspace.git/tree/lookandfeel/contents/components/UserDelegate.qml#n102

omits the precision specifiers required for GLSL/ES compatibility, see:

http://doc.qt.io/qt-5/qglshaderprogram.html#writing-portable-shaders

Running e.g. sddm-greeter on a GL/ES 2.0 plattform results in the following error message:

Jul 15 09:11:11 susetest sddm-greeter[6178]: QOpenGLShader::compile(Fragment):

0:6(2): error: No precision specified in this scope for type `vec4'
0:7(2): error: No precision specified in this scope for type `float'
...

First one refers to "uniform vec4 colorBorder;", next to "float blend = 0.01;".

The vec4 {colorBorder, colorEmpty, colorSource} should be "lowp" (sufficient for color representation), all other (used for coordinate calculations) should be "highp".