Post your Sep17log.txt as an attachment here.
If Cumulus says it has an error, then that file does have an error, and the FAQ tells you how to solve it, so we need to be sure you are not misreading your file. Proof readers exist as a job because we often see what we expect, not what is actually there.
Reinstalling Cumulus does not correct an error in a log file. I'm not being unkind, but you sound as if you are in a panic mode, and that is not the best for solving problems. Even if I can't solve your problem (and I'm working one step at a time), there are plenty of others to offer their help. Cumulus outputs a lot of diagnostic information and it might be necessary to look at that.
Also are you letting either your pc or the USB go to sleep while Cumulus is running? There are a lot of posts on the forum about those sort of problems, so far I have focussed on trying to rectify the problem with your file, but you may need to find the cause too.
shaunfish wrote: the program will insert some strange rainfall figures such as 200mm/hr.
The wrong rainfall figures is probably unrelated to the log file issue, but it certainly is not a feature of Cumulus (as Steve says in other posts, Cumulus has incorporated a lot of features to try to reject incorrect figures), it might be an indication of a faulty set bit in your weather station output message or it might indicate your computer had a hiccup when reading the input. Again both the FAQ and the forum make lots of references to strange or rogue figures. Editing rainfall is fairly easy, see the FAQ. You can't expect your Watson to be perfect, even professional weather stations have to manually edit out some rogue figures from time to time.