WMS map doensn't open in QField, but does in desktop QGIS

[Dutch5972]

Hi,
Since a year I am using a WMS backround map in QField. For using this map I have a subscribtion with its developer. Till recently it worked fine on both QGIS and QField (Samsung Android). But suddenly it stopped working in QField although it is still working in QGIS. The provider mentioned no other customers reported any problems with using this background map.
I reïnstalled the latest QGIS version (Lima), the latest Qfield sync plug-in and the newest version of QField. The 'Save path' setting in project properties is set in 'relative'.
In QField in the layerinfo it says that the layer is invalid, which might be due to a network problem, an unknown file or wrong configuration of the project. But the project settings have not changed and on QGIS the WMS layer is working properly. Other standard WMS background maps and aerial photos are working fine though.

Does anyone know how to tackle this issue? Thanks in advance!