Graphics problems with LyX 2.3.4.2 on Linux Mint

[Graeme]

I currently use Linux Mint 18.3. Recently I upgraded to LyX 2.3.4.2 
using the PPA available at
http://ppa.launchpad.net/lyx-devel/release/ubuntu xenial main

Since this upgrade there have been problems with the display of EPS and 
PDF graphics files in the main LyX editor window, and with the export of 
files containing PNG and JPEG bitmap images.

These problems are not evident with LyX 2.3.4.3 on Windows 10, nor were 
they present with LyX 2.3.3 on Linux Mint 18.3.

Specifically:

1. When including a graphic with either EPS or PDF format, the graphic 
is not displayed in the main LyX editor window, but instead there is an 
outline box with the EPS or PDF filename, and below it the error 
message: Error converting to loadable format.

The graphic does appear correctly in a PDF file exported using either 
ps2pdf or pdflatex.

2. When including a graphic with PNG or JPEG format, the graphic is 
displayed correctly in the main LyX editor window, but when attempting 
to export the LyX file to Postscript, PDF (using ps2pdf), or even just 
plain LaTeX, an error pop-up window appears with the title:
LyX: Cannot convert file
and the message:
No information for converting png format files to eps. Define a 
converter in the preferences or No information for converting jpg format 
files to eps. Define a converter in the preferences.

The PNG and JPEG graphics do appear correctly in a PDF file exported 
using pdflatex, but attempting to export with ps2pdf fails.

When exporting the Progress Log window shows the following messages for 
a graphic in PNG format:

> python -tt "/usr/share/lyx/scripts/convertDefault.py" png "/tmp/lyx_tmpdir.fGfmcihv6288/lyx_tmpbuf0/2_home_graeme_Documents_GraphicsTestCircle.png" eps "/tmp/lyx_tmpdir.fGfmcihv6288/lyx_tmpbuf0/2_home_graeme_Documents_GraphicsTestCircle.eps" 
> 
> convert: not authorized `eps:/tmp/lyx_tmpdir.fGfmcihv6288/lyx_tmpbuf0/2_home_graeme_Documents_GraphicsTestCircle.eps' @ error/constitute.c/WriteImage/1028. 
> 
> /usr/share/lyx/scripts/convertDefault.py ERROR 
> 
> Execution of "convert" failed.support/Systemcall.cpp (276): Systemcall: 'python -tt "/usr/share/lyx/scripts/convertDefault.py" png "/tmp/lyx_tmpdir.fGfmcihv6288/lyx_tmpbuf0/2_home_graeme_Documents_GraphicsTestCircle.png" eps "/tmp/lyx_tmpdir.fGfmcihv6288/lyx_tmpbuf0/2_home_graeme_Documents_GraphicsTestCircle.eps"' finished with exit code 1 
> 
> Error: Cannot convert file

I get similar messages for a graphic file in JPEG format.

My suspicion is that BOTH the problems above may be linked to the use of 
ImageMagick within convertDefault.py, but I am puzzled by the warning 
message
"convert: not authorized"
in the fragment above, suggesting it may be something to do with file or 
folder permissions.
However, this does not explain why everything seemed to work OK with LyX 
2.3.3 on Mint 18.3

On Mint 18.3 the system-wide installation of ImageMagick is:
ImageMagick 6.8.9-9 Q16 x86_64 2019-11-12

On Windows 10, the LyX2.3.4.3 installer comes bundled with a later version:
ImageMagick 7.0.9-19 Q16 x86 2020-01-26
which explicitly lists built-in support for gslib and ps, but 
ImageMagick 6 does not list these among its "Delegates".

I would welcome advice on how to fix these problems.

Graeme