Something weird with a sign field and my surface pro 7

Hi everybody,

When i use a sign field on my PC and my mouse, no probleme signature is ok

But when i'm trying on my surface Pro 7, nothing happens. I mean it's like the ink is white (whatever i use, my finger or surface pen). when i click OK, an 'uploading file" message appear, so i think my sign is ok but "invisible"

i have no idea how to fix this......

Thanks in advance

Julian