By now the error has changed; could it be that UltraVNC keeps releasing new Version 1.2300 with different hashes?

You have ultravnc v1.2240.0.20190403 installed. Version 1.2300 is available based on your source(s).
Progress: Downloading ultravnc 1.2300... 100%

ultravnc v1.2300 [Approved]
ultravnc package files upgrade completed. Performing other installation steps.
File appears to be downloaded already. Verifying with package checksum to determine if it needs to be redownloaded.
Error - hashes do not match. Actual value was '5168CD279B36141B4B5B21CAC449FD4738A0E7C281D19BE99B3EF22968D77B7E'.
Downloading ultravnc 64 bit
from 'https://www.uvnc.eu/download/1230/UltraVNC_1_2_30_X64_Setup.exe'
WARNING: C:\Users\jeroenp\AppData\Local\Temp\chocolatey\ultravnc\1.2300\index.php is of content type text/html; charset=utf-8
Download of index.php (-1 B) completed.
Error - hashes do not match. Actual value was 'D0030DCF7A58166CC96FF5F258D5E6F179F94F412456CD0CE7223561532A3AB2'.
ERROR: Checksum for 'C:\Users\jeroenp\AppData\Local\Temp\chocolatey\ultravnc\1.2300\index.php' did not meet 'fe3d1135ae0e7b72394a6f3cc137282cb5e6382a55b5ceee72140d28f5ffe961' for checksum type 'sha256'. Consider passing the actual checksums through with --checksum --checksum64 once you validate the checksums are appropriate. A less secure option is to pass --ignore-checksums if necessary.
The upgrade of ultravnc was NOT successful.
Error while running 'C:\ProgramData\chocolatey\lib\ultravnc\tools\chocolateyInstall.ps1'.
See log for details.