I have solved my problem:
On my design I did not intend using the internal ADC and did not connect the VDDA to VDD as I thought that VDDA was only used for the internal ADC, my mistake!
The VDDA is also used for the following:
VSSA, VDDA = 2.0 to 3.6 V: external analog power supplies for ADC, Reset blocks, RCs and PLL (minimum voltage to be applied to VDDA is 2.4 V when the ADC is used). VDDA and VSSA must be connected to VDD and VSS, respectively
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