The problem with the above error is that you can't tell when it popped up. You might, for example, have a start up script that attempts to modify data in a table for which the user does not have write privileges--maybe a global field? and then you'd get that error. If you have advanced, you can launch FileMaker Advanced, enable the script debugger and then open the file in order to watch any such script run and see which step trips this error message. You may find that you need to allow write access to a utility table and deny write access to all other tables in the file.
Yes, you do need to set up identical credentials in each file that the current file connects to. This is a strong argument for keeping the total number of files to your solution to a minimum as managing credentials in a multi-file system is a major headache. There are also ways to set up a script to manage account changes such that you can use a script to create or change accounts identically in a set of files by performing a script to do this in each file. (but unfortunately, this scripted approach cannot be used for externally authenticated accounts. )