Hi standal,
I am sorry that your meter is not working normally. But before you throw it try a few things. And I would suggest you to still keep it even if it doesn't work; you can repurpose some of its parts.
The good thing with this meter is that it is based on a commercially available all in one chip so you can try to use its datasheet for troubleshooting. Because it will be using something similar to the proposed design from the datasheet.
I don't own this meter so what I am writing here is from the pictures and the datasheet that I have found on the internet. So, there could be some mistake.
Try the following:
Remove C7 (its under the display, below the main IC) and see if meter's behavior changes. It looks like a bypass to ground cap in the main V (input socket) line. If it is leaking it could cause such behavior, I assume.
If that doesn't help, start disconnecting the main IC from the outside world. This will help you determine if it is OK or not.
As a first step, remove R20 (820 ohm, I guess), it is right above C7 and see if it makes any difference with NCV, Hz and Cap. It is the input to main chip for these functions.
If current ranges are giving any problem, remove C3 and check again. After removing C3, remove L2 to disconnect that input completely.
C4 and L4 are also input to the main chip for R function, I guess. It works along with R20 line.
Then as last resort, you can remove either any one of R1a to R1d resistors, or L5 inductor (along with C7). It will disconnect the V input to the meter.
I hope it'll help.