The photo that José included looks to be the type of can with the screw-on cap.  These containers normally have the metal seal that has to be pried off when the cap is removed.  Maybe this seal had failed to do its job.

I had a quart can (with screw on cap) of Brodak clear that dried up.  I used some of the dope, put the cap on, then put it aside for maybe a year.  When I opened it the dope was the consistency of putty.  I figured that the cap had not sealed very well even though I needed pliers to get the cap off.  This clear was not the Crystal Clear but was the clear that had a slight amber tint.

I never had such a problem with the cans with the lid that is held on with the tabs and then has to be pried off. 

If the can was never opened and the seal intact, then maybe a formulation error is to blame.

As Ty mentioned, you can use Randolph dope as it is compatible with Brodak dope.  I have some Randolph clear, both tautening and non-tautening.  The label on the tautening dope says that it is not to be used on any polyester fabric.  I guess this would include polyspan.

Thanks TY, Tim and Gerald for your replies.

Yes, the can's have screw on cap, but for sure the seal had failed.    

However, thanks too for yours opinions and recommendation of Randolph products.
Like is an American products, I've no idea the Brodak dope and similar are made by themes  

Already make a search and find Randolph products here in Europe. Its good have good products near us.

Cheers
José