I experienced the same thing, when I started vaping. Also had extremely dry sinuses, mouth and throat. When I could get the phlegm out of my throat, it was a very viscous clear, slimy consistency. The mornings were the worst, my nose / sinuses would be stopped up and I'd end breathing through my mouth, throughout the night, which dry out my throat even more. I tried increasing fluids but it seemed like water would dry out my mouth even more. (Sweet tea helped a little) and sometimes eating a pickle can relieve the mouth / throat symptoms.
I read about PG allergies and thought that may have been the problem, so I started buying high VG juice and it got worse.
In the end, what fixed it for me was lowering the VG and increasing the PG. I now vape 50pg/50vg ........ at most I'll vape 40pg/60vg if I up the VG any higher than 60% I start getting the dry throat, stopped up sinuses, phlegm issue again.
Another thing to consider:
If you are buying your juice at a local shop and they're mixing it when you order, let that juice sit in a desk drawer for a week before you vape it. The ingredients in the fresh juice are melding / marrying / mixing together and becoming a new compound. Sometimes while this is happening the vape can be harsh and irritating.