About URL encoding in web viewers
FileMaker Pro evaluates a web viewer's web address, encodes the URL, then tries to load the web page using this URL. FileMaker Pro uses the following URL encoding rules to fix only the most common problems:
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ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
0123456789-_.~!*'();:@=+$,/?[]
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Note  All of the characters listed above are Unicode characters. When a character is encoded, both bytes are encoded separately (unless the first byte is 00), as described by the URL encoding standard. See http://www.w3.org/addressing/.
Because FileMaker Pro cannot correct all possible URL encoding problems, test your web viewer to ensure that its URLs are correctly encoded.
Related topics 
Defining a custom web address
Troubleshooting layouts with web viewers