As Jerzy correctly pointed out, it's vital to indicate which version you're using (hence the notice in red that is displayed when you're posting a new topic...).
The full version number is displayed in the software (go to Help - About - this is where you will find version info for virtually all Windows applications).
I suppose there's no way of mending this cumbersome piece of expensive software once it breaks?
How much did you pay for MultiTerm, then?
Seriously: I don't think you can 'break' the software.
I cannot make any new entries into my termbases, because the form to type the entries into no longer appears.
Which is a user-defined form called an input model. What happens if you select the default input model?
It always did take a long time, with each individual entry, before the "Applet controller" started up, that made the form appear. In the past few days it would not work any more until I re-booted the computer, which is usually terribly inconvenient, as start-up is so interminably slow and it takes up to half an hour to get everything going again, and now, even on re-boot, the Applet controller will not start.
TBH this sounds like the problem lies with your system rather than with MultiTerm... what operating system are you using? Which system gave you the message about an applet controller? Are you using firewall software?
A Windows XP system should not take much more than a minute to start up...
I did not have all that many entries in these termbases yet, so they can't exactly be overloaded, but I guess it's just my tough luck that it broke at such an early stage - or is there a possible way to fix it, without going to the expense of buying it all over again?
Why would you need to buy it again? Assuming there's something wrong with the installation (and we haven't ascertained that yet), why couldn't you reinstall?