This is a minor bug:
If I open a Fleet's Set Speed input in the Naval Organization window under the Fleet Tab and I do not change the speed but just select OK as opposed to closing via the X, it throws a 'Function #939: Input string was not in a correct format'.
Got the same thing, I assume it's the same reason for you as me.
It seems be another one of the "Aurora is very picky about formatting" issues.
When I opened that menu the speed was shown as 1 300 instead of 1300. Changing it to 1300 made the error go away.
Permanent fix was to change another formatting setting on the PC. The digit grouping this time.
Edit: Would be nice if we did not have to do this all the time Steve. For when the bug torrent slows down a bit