In addition to my above post: I have been having Designer running for long times, so I didn't notice at first, but it seems that the problem in question does not appear only on the first time the application is launched. On the second launch, as soon as the application is loaded (or, I suspect, as soon as the fonts are loaded), the problem is in full effect again, and I cannot do much else than to boot my computer. I just downloaded the newest beta (1.8.4.665), and the problem exists with that as well in the same way. On the first launch, everything works; on the second, some or all text in various applications, including Windows desktop and both the user interface of the Designer and the document being open in it, do not appear or are unreadable.
My Windows 10 is 64-bit, version 1909 (UI build: 18363.900). Graphics card: NVIDIA Quadro K2100M.