"So probably a software problem after all." I'm amazed you say that when the experiment showed that with abundant RAM, there is no problem.
Because there is so much to tweak and update, I am back in the 32bit system now, and found the PC (Windows ?) telling me it could not read the drive - I fixed that by reducing the RAM to its original 4Gb - so I interpret that windows thought I had moved the disk to a different PC.
In the end, the only 'serious' downside was the cost of a new Kleptomania - cheaper than YOU quoted.
The Windows 64 bit is v20H2.
You MIGHT be interested that the Sendleap desktop app refused to recognise my Google credentials - EVEN THOUGH THE WEB APP WAS ALREADY OPEN. Sheer persistence eventually brought it to its senses, but Google sent me an email that my account had been RECOVERED - the fantasies these robots devise !
There is a lot of tweaking etc to do before the 64bit will replace the 32bit, but I am so impressed by the 'smooth operation' instead of intermittent faults, that I will make that effort.
'Something else that is dear to me' ALSO works with flying colours in 64bit, so I'm going to have no more reservations about the 'upgrade'.
As stated before, the original issue of corrupted pasting into Telegram, I found a work-around for; and through your patient investigation we appear to have fingered the culprit causing THIS PC to misbehave - lack of memory; I already had the hardware (RAM chips) and software (a 64bit system) to remedy the situation - which for now I consider successfully solved.
Once more, many thanks for sticking with it till the (bitter)sweet end.