The majority of mobile browsers only support the old 2009 Flexbox properties. Your code should look like this:
.postpreview {
display: -webkit-box;
display: -moz-box;
display: -ms-flexbox;
display: -webkit-flex;
display: flex;
}
.psto {
-webkit-box-flex: 5;
-moz-box-flex: 5;
-webkit-flex: 5;
-ms-flex: 5;
flex: 5;
}
.datencomments {
-webkit-box-flex: 2;
-moz-box-flex: 2;
-webkit-flex: 2;
-ms-flex: 2;
flex: 2;
vertical-align: top !important;
margin-top: 15px;
}
@media screen and (max-width: 380px) {
.postpreview {
display: -webkit-box;
display: -moz-box;
display: -ms-flexbox;
display: -webkit-flex;
display: flex;
-webkit-box-orient: vertical;
-moz-box-orient: vertical;
-webkit-box-direction: normal;
-moz-box-direction: normal;
-webkit-flex-direction: column;
-ms-flex-direction: column;
flex-direction: column;
}
}
Also note that the default flex-direction is row, so there's no need to specify it unless you're overwriting a previous flex-direction setting.