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Can you please send me a crash report? See http://feedback.textasticapp.com/topic/48133-how-do-i-send-crash-reports/

Are you absolutely sure you are using version 3.1? I tested it with much larger directories on Dropbox and it didn't crash for me.
Is this with Dropbox or WebDAV or with FTP downloads?

Just a few hours ago I discovered a new bug with FTP downloads. Please try to limit the number of concurrent connections to 1 in the FTP configuration screen and try to download again.

The bug happens when parsing file listings from the FTP server in concurrent threads. 

Let me know if that makes any difference for you.
I could reproduce the problem now. It will be fixed in the next update.
This will be fixed in the next update.
Hi, the FTP server sends this error message:

550 page_01a.htm: Permission denied 

So it looks like the user you used to login doesn't have permissions on the server to create or write files in this directory. Changing file permissions on the server should fix this.

Please try to enable the "Debug Log" setting and have a look at the log file.

What is the exact error message?
For syntax highlighting, there is a also a DocBook TextMate bundle you can try at https://github.com/weppos/docbook5-tmbundle

Here's how to use custom TextMate bundles with Textastic: http://www.textasticapp.com/v3/manual/lessons/How_can_I_add_my_own_syntax_definitions_and_themes.html
Did you see the following entry regarding Docbook preview?

http://feedback.textasticapp.com/topic/80566-how-can-i-preview-docbook-files-in-textastic/

Thanks for reporting this. I'll try to reproduce and fix it.