Transfering modern HTML files via a FTP program can lead to the corruption of these HTML files, which then leads to the results as you experience them here.
The cause of this problem is, that many FTP programs transfer your HTML files in ASCII and not in binary mode. Due to the way this mode works, one eighth of your HTML files simply gets lost during transfer, since the ASCII standard is only using 7 instead of 8 bits per byte. The differentiation between ASCII and binary mode has historic reasons and with today's average bandwith does no longer give you any real performance advantages, but mostly problems when transfering modern file formats. The solution to the problem is to set the transfer mode of your FTP program to "binary" (and not "automatic" or "ASCII"): all files are then transfered correctly and completly and all umlauts and special characters are displayed correctly.