Document "document" could not be saved... But... !!!!

Hello

I've recently replaced my iMac and I'm now running Yosemite 10.10.5

I'm using Numbers '09 to edit a spreadsheet, and this old version of Numbers has been working absolutely fine since the day I installed it on my Yosemite iMac.

I have some financial spreadsheets that I store on an encrypted Disk Image.

Today, I have come across something really really odd. I can open the spreadsheets, but I can't save them.

It says "Document ...name... could not be saved.

But here's the odd thing. If I press Option to get the Save As menu back, I can save the file, with the same name, back where it came from. And then I can't "save" it again.

And of course, I can Save As a new file, and then, while that file has been written to disc, I can make a change in Numbers and the file won't save again.

So in summary, I can't "Save" but I can "Save As"....

So it's not read/write permissions on the file, and besides it was OK yesterday. I'm unmounted the disk image and remounted it, and that hasn't helped either.

Next step is to reboot my Mac, but I've got something running in the background that I don't want to stop at the moment.

Has anyone else seen this weird behaviour???

DerekH