Haven’t tried to do this myself. Looks like a bug.
Have you tried to do the same from the CLI (and without using that Web site), to be able to tell if it’s the site or CP s/w issue?
I don’t have the CLI installed but I tried other tools
https://coinb.in/#sign can sign a hex string from http://redeem.bitwatch.co/. If it contains dozens of inputs, doesn’t matter, it successfully signs (beware: you need to insert your private key - i only dared to use a test account).
Then I tried once more, but only with one input this time. Again, the signed tx from coinb.in could not be pushed. In Counterwallet I could sign and push it successfully.
The signed hex looked slightly different in Counterwallet and coinb.in but I could not copy it from CW’s GUI, hence not test if it could be pushed from one of the above mentioned sites.
If you feel there’s a bug in CP feel free to submit it on Github.
OT: if you install Virtual Box with a minimal Ubuntu 14.04 instance you can install counterparty-client and configure it to use public servers (for both Bitcoin Core and counterparty). I can’t remember if there are any public Counterparty testnet servers, though.
Edit:
you can also install counterparty-gui at the same time, if you want to be able to use a GUI for simple operations (Send, basically, although on Ubuntu (and Linux in general) there are too many Python dependencies that need to be installed so I wouldn’t recommend this for Linux - only OS X and Windows)
You can also install counterparty-client (and GUI) on Windows without any VM’s, and use it from Windows shell. This is quick and easy, but the Windows shell is a nightmare.