You can sign up for the Textastic 6 beta at http://goo.gl/forms/piRaSmBzmM. It has full support for the iPad Pro and split view multitasking.

Two days ago, an Apple certificate that is used to sign Mac App Store apps expired. This may require a login to the Mac App Store, so that a new receipt is generated that is signed with a new certificate. This should happen only once per machine.

Thanks, I can reproduce the issue. I'll fix it in an update.

Textastic 6 will support split view multitasking. You can sign up for the beta here: http://goo.gl/forms/piRaSmBzmM

It looks like the OneDrive file provider implementation is buggy. I had similar problems with it before and it was basically unusable. I hope they fix it soon.

Other apps like Working Copy or iCloud Drive work fine.

You can enter arbitrary URLs when you open the preview screen and switch from Local to Remote, so this is necessary.

I could reproduce the error with the ownCloud Demo installation at https://demo.owncloud.org/

I do get the errors you describe in Textastic and I also tried it with Byword. When using Byword, I get the same error messages on the debug console. The only difference is that Byword doesn't present the errors to the user while Textastic does. You can check this for yourself with the "iOS Console" Mac app http://lemonjar.com/iosconsole/

So, this really looks like a bug in the ownCloud file provider extension. I have debugged everything related to saving and can't find a problem in Textastic's code.