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Thank you for the replies -- I received two messages. I will contact the LyX mailing list. Thank you once again. Gyorgy.

Dear List members,

I have a problem with LyX 2.3.0.
Details:

1. Operating system:
Windows 10x64.

2. MiKTeX installation -- MiKTeX 2.9 (32 bit):
The last one from July 18, 2018.
(As a matter of fact I updated my MiKTeX installation
this morning -- before installing LyX 2.3.0)

I removed LyX 2.2 ...