Two things to check to start.
For the files you are trying to access, did you:
a) Open the files with FileMaker Pro (Not server for this task)
b) Select File | Sharing | Filemaker Network...
c) Select each such file that you have open and then click "All users"
If that's not the issue,
check your .PEM files.
For More Information see:
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip