I think I saw in another thread (can't recall where) that typing a single space at the end of the serial number resolves that, and that it's a known bug.

I'll try to find the relevant post/thread...

Here it is - HELP - My serial number is being rejected from posersoftware.com. Not quite what I thought, but maybe worth a try ?

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The 3Dcheapskate* occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.

*also available in ShareCG, DAZ, and HiveWire3D flavours (the DeviantArt and CGBytes flavour have been discontinued).