This response is in lieu of anything more helpful, which is sorely lacking from my repertoire:
- have you restarted the iOS device, and does the error persist?
- have you installed DTTG 3.5.2, which was made available in the past few hours?
- have you tried from another network?
- do you have enough space available on Dropbox?
In addition: a similar error message was fixed in the past by running a service command; see here for details.