The problem on High Sierra seems to be fixed in macOS 10.13.2. 

See also this tweet: https://twitter.com/Catfish_Man/status/938489343605276677

I doubt that it is related.

I have the same problem on High Sierra on my MacBook Pro when I use Cmd-O to show the Open dialog. 

This is a bug in macOS High Sierra. I already reported it to Apple. It is hopefully fixed soon. I can reproduce the same problem in Apple's own TextEdit app. It does not happen on macOS Sierra or earlier.

Yes: First, make sure to add the Working Copy repository to Textastic using Drag & Drop as shown in the video at https://www.youtube.com/watch?v=Z0gyGwsF8vI

Navigate into the repository you just edit and edit the file. Then tap on the "globe" button to open the remote file transfer screen. 

Upload the file to your test server. You only need to do that once. After the first upload, the file is linked to the remote location and you can use the share button to upload it to that locatiation.

Please have a look at the app "Marked 2". You can drag the file icon from Textastic's window title bar onto Marked 2's Dock icon to quickly preview Markdown.

Hello, which error messages do you get when you try to upload files?

This should be fixed in Textastic 6.4.2.

This should be fixed in Textastic 6.4.2.

This should now work with the latest versions of Textastic (6.4.2) and Working Copy (3.1.7).

Weird, I'll look into it.