Hey Octavian. Welcome to IOTA! I'm sorry that your introduction to the platform was marred by this issue; let's see if we can work it out!
It is possible for a seed to be less than 81 trytes in length. The closer you get to 81 trytes, the more secure your seed will be, but the underlying crypto functionality will automatically pad it to 81 trytes anyway, so there is no technical limitation that prevents you from using a shorter seed.
There are a few issues with the Wallet app at the moment that cause it to sometimes show an incorrect 0 balance, so the best way to verify that your seed is correct is to generate some addresses and see if it is able to produce the same address as the one you sent your Bitfinex withdrawal to.
I'm not super familiar with the way the Wallet app works. You might be able to accomplish this by deleting the Wallet's local database, logging in with your seed and generating a bunch of addresses — I've heard that some users are able to recover their balance this way.
Alternatively, if you are comfortable on the command-line, you can generate addresses using the Python or Node.js IOTA API library. This will tell you definitively if you have the correct seed.
If you do not have the correct seed, I recommend taking a look at this thread. He had a similar problem, and he was able to recover his IOTAs successfully in the end: