This problem was be fixed in FN 7.5.3. Please run software update to download and install the latest version.The problem only occurs on new checking accounts being setup for CheckForm Navigator(CFN). The problem does not occur on editing existing CFN settings.
One way to work around the problem is to create the CheckForm Navigator settings file from a copy of another CFN setttings file. Copy a file named 'lsr999.chk' to 'lsrXXX.chk', where 999 is the account number of an existing checking account on CheckForm Navigator and XXX is the account number of the new checking account. Now change the CFN Settings just like normal for the new checking account for the Bank info.
Another way is to close FN and leave the CFN setting dialog box open. So now the account file is not in use by FN and the error 70 will not occur.