I get an error message "Error checking for new version. SSL/TLS support is not compiled!" (screenshot attached) whenever I choose 'Check for updates' from Help menu.

To resolve the issue I replaced original libeay32.dll and ssleay32.dll with 64 bit versions from https://indy.fulgan.com/SSL/?C=N;O=D (openssl-1.0.2t-x64_86-win64.zip to be specific).
Could it be that installation package for 64 bit Transmission Remote GUI is bundled with wrong type of OpenSSL libraries?
Interesting...
Here's the info of dlls in the repo, as you can see the file and machine type will be different:
~/transgui $ exiftool setup/win/openssl/*.dll | grep '^\(File\|Machine\) Type '
File Type : Win32 DLL
Machine Type : Intel 386 or later, and compatibles
File Type : Win32 DLL
Machine Type : Intel 386 or later, and compatibles
~/transgui $ exiftool setup/win_amd64/openssl/*.dll | grep '^\(File\|Machine\) Type '
File Type : Win64 DLL
Machine Type : AMD AMD64
File Type : Win64 DLL
Machine Type : AMD AMD64
This is the inspect result of the dlls of 64bit release, looks correct to me:
$ wget -q https://github.com/transmission-remote-gui/transgui/releases/download/v5.18.0/transgui-5.18.0-setup_64bit.exe
$ innoextract transgui-5.18.0-setup_64bit.exe
Warning: Unexpected data while converting from Windows-1252 to UTF-8.
Extracting "Transmission Remote GUI 5.18" - setup data version 5.6.0 (unicode)
- "tmp/idp.dll" [temp]
- "app/transgui.exe"
- "app/LICENSE.txt"
- "app/README.md"
- "app/README.txt"
- "app/history.txt"
- "app/lang/transgui.be"
- "app/lang/transgui.ca"
- "app/lang/transgui.cs"
- "app/lang/transgui.da"
- "app/lang/transgui.de"
- "app/lang/transgui.el"
- "app/lang/transgui.en"
- "app/lang/transgui.es"
- "app/lang/transgui.fi"
- "app/lang/transgui.fr"
- "app/lang/transgui.hr"
- "app/lang/transgui.hu"
- "app/lang/transgui.it"
- "app/lang/transgui.ko"
- "app/lang/transgui.lt"
- "app/lang/transgui.lv"
- "app/lang/transgui.nl"
- "app/lang/transgui.no"
- "app/lang/transgui.pl"
- "app/lang/transgui.pt"
- "app/lang/transgui.pt_br"
- "app/lang/transgui.ro"
- "app/lang/transgui.ru"
- "app/lang/transgui.sv"
- "app/lang/transgui.template"
- "app/lang/transgui.tr"
- "app/lang/transgui.uk"
- "app/lang/transgui.zh"
- "app/lang/transgui.zh_tw"
- "app/libeay32.dll"
- "app/ssleay32.dll"
Done with 1 warning.
$ exiftool app/*.dll | grep '^\(File\|Machine\) Type '
File Type : Win64 DLL
Machine Type : AMD AMD64
File Type : Win64 DLL
Machine Type : AMD AMD64
Currently we didn't receive other feedback like this issue.
version 5.18 32/64 bit, Windows 10x64, SSL / TLS does not work.


Same issue on fresh Win10 Install:

I uninstalled the 64 bit version. 32 bit version works fine.
I get the same error when trying to update geoip database. Running 5.18 on Linux Mint 64bit
The standalone Windows 64 bit ZIP files are missing the two dll files hence the error on windows.
The setup has them but not the zip file.
You will need to include them when packaging the standalone 64 bit zip versions.
@Revenger The zip tarball is intended to not include them, please take a look at README.
I had same issue.
I solved the problem like this:
Download the OpenSSL library indy.fulgan.com/SSL/openssl-1.0.2q-x64_86-win64.zip and place it in the Transmission program folder (in %ProgramFiles%\Transmission Remote GUI). You need to replace ssleay32.dll and libeay32.dll. It helped me to update the Geoip database.
Apparently, OpenSSL version 1.1 has renamed the DLLs to this:
libssl-1_1-x64
libcrypto-1_1-x64
But because transmission-remote-gui looks for ssleay32 and libeay32, I believe it cannot find the dll
I had this problem too (non working update, and geoip download..). What helped was to download and extract:
https://indy.fulgan.com/SSL/Archive/Experimental/openssl-1.0.2o-x64-VC2017.zip (in parent web path are also other packages for other OS)
and copy libeay32.dll + ssleay32.dll to C:\Program Files\Transmission\transgui and restart TransGUI app.
Can developers @PeterDaveHello @leonsoft-kras @antekgla please include these files by default? I am having TransGUI 5.18.0 Windows 10 64bit.
Same problem with U 20.04, apparently need the two files. Any 'official' versions? (Vaguely remember running into this years ago. Both are long gone now.)
Doesn't seem right to have features broken right out of the box. Looking for at least a link to the 'real' versions, thanks.
Just stumbled across this problem, turns out that the openssl dlls that are installed with the 5.18 x64 installer have a dependency to MSVCR120.dll. In other words, they were built with Visual Studio 2013 and require the Visual C++ 2013 redistributables to run. They can be found here: https://support.microsoft.com/en-us/help/4032938/update-for-visual-c-2013-redistributable-package
I would suggest shipping with openssl libraries linked with the static VC++ runtime (/MT) to avoid this problem in the future.
Most helpful comment
Just stumbled across this problem, turns out that the openssl dlls that are installed with the 5.18 x64 installer have a dependency to MSVCR120.dll. In other words, they were built with Visual Studio 2013 and require the Visual C++ 2013 redistributables to run. They can be found here: https://support.microsoft.com/en-us/help/4032938/update-for-visual-c-2013-redistributable-package
I would suggest shipping with openssl libraries linked with the static VC++ runtime (/MT) to avoid this problem in the future.