However, the early Ming government enacted a law, later rescinded, which forbade Han Chinese to learn the tenets of Tibetan Buddhism. There is little detailed evidence of Chinese—especially lay Chinese—studying Tibetan Buddhism until the Republican era (1912–1949). Despite these missions on behalf of the Hongwu Emperor, Morris Rossabi writes that the Yongle Emperor (r. 1402–1424) "was the first Ming ruler actively to seek an extension of relations with Tibet."
What years did the Yongle Emperor reign? 
1402–1424