Interpretation of the following mass spectrum
I was on to the wrong structure. My main issue with the carboxylic acid was the fragment m/z=60 that it would produce by McLafferty. I didn't have that, so I thought that left me with t-Bu-COOH, which was the only structure with no $\gamma$-hydrogens. Besides I couldn't explain the m/z=74 peak with an acid. On the other hand I couldn't explain the M-17 peak by ethylpropanoate.

But of course, if the carboxylic acid has a methyl substituent at the $\alpha$-carbon, then that methyl group would also be part of the McLafferty fragment, therefore resulting in m/z=74!

The spectre is of: 2-methyl-butanoic acid

M-17 and M-45: 

Alpha cleavage of carbonyl => loss of hydroxyl radical or carboxyl radical



m/z = 74:

Loss of ethene by McLafferty rearrangement



m/z = 45:

$\ce{COOH^+}$ fragment



M-15 and M-29:

Loss of methyl radical or ethyl radical



m/z = 29:

Ethyl carbocation