Mick Hardy
I recently spent a bit of time uncovering the cause of a 400 Bad Request error on a customer's computer in Croatia.

It turns out his keywords were too long. There appears to be a maximum number of characters allowed in the keywords field. It isn't the total number of keywords, just the total length of the keywords.

Copy and paste your keywords into Microsoft Word or http://www.wordcounter.net  and decrease the total characters to 425 or lower.

Mick Hardy attached the following image(s):
Mick Hardy
This is documented after all. There is a 500 character limit and a max length per keyword of 30 characters. I have a few forced keywords at the start, which lowers it to 425.

I’ll ensure to enforce this when entering data. Also, keywords with spaces are enclosed in quotes which counts against the overall total.

12 34,56 78 becomes “12 34”,”56 78” with a total of 15 characters.
Mick Hardy
I've finally released the latest version, which resolves this issue.