Could you please describe step by step how exactly you performed the server upgrade?
Are you using MSI packages or zip?
How are you running the server, via a service or starting it manually?
Thanks.

I always use the ZIP version. I have the server running as a service. These are the steps:

1) Open a command prompt with administrator privileges
2) Stop the service; service exits successfully
3) Unzip the new version, copy the new version files (replacing the old 10.0.15 files) to the server folder (c:\mysql in my case)
4) Start the service; service starts successfully
5) run mysql_upgrade -u root -p it prompts for the root password, then runs the mysql_upgrade (table checks, etc.)

Now with 10.0.16, it reports the error I described after entering the root password. This has worked correctly in all previous versions of mariaDB I've used and also MySQL before I switched to mariaDB.

Hi,

Is the problem persistent?

I tried to do the same, but couldn't reproduce it.
The error means that mysql_upgrade could not create a file in the temporary directory. The directory can be either configured in the ini file for clients, or it can be the default one (on my Server 2012 it's C:\Users\<username>\AppData\Local\Temp.

Could you please provide your ini file or check yourself whether tmpdir is set there, and make sure that the tmpdir (either the configured one or the default one) is actually writable?

I tried it on two other servers and it works correctly. I believe this can be closed.

According to the above note, closing as "Cannot Reproduce". If you have new information later, you can comment to re-open the report.