I had (and still have to an extent) the same problem, I fixed it briefly by removing the supply pipe to the t piece and working the washers, the stuff that came out smelt foul.
When I filled the bottle with very hot water it bubbled up like a swamp, worked ok for a while though.
I'm thinking it's poor quality screenwash breaking down into jelly, my fix is going to be to drain it again, leave a bottle of white vinegar soaking in there for half a day or so, flush with boiling water then refill with quality screenwash.
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