It was Aris day 4 - right after Zhokar showed off his "fighing gear".
I managed to pinpoint the issue:
1. On Aris Day 4, skip to being tied up in the Warehouse, and have Zhokar's dialog on the screen
2. Open Supplies menu
3. Click the Repair Hammer (this causes a choice menu to show up "behind" the inventory screen so you can't really see anything)
4. Click Return and click through the merchant dialogue
5. Click Return again
Clicking Return causes merchant dialogue to trigger that "you don't have enough ouros for that". After pressing it again, it is again interpreted as clicking a Hydronetics Scroll at a merchant, so it starts dialogue of whether to buy it (the choices appear behing the inventory screen so you can't tell what to click). It's not possible to exit the "Buy Hydronetics scroll? -> Oh sorry you can't afford that" loop, so you have to Alf+F4 the game and reload a save to continue.
This is the best reproduction of the bug I could come up with.