OFDM sensitivity to sampling time offset (with half of time interval between samples) In simple QAM modulation symbol synchronization is crucial: the samples need to be taken (or interpolated and resampled to ) exactly corresponding to the time when the symbol was emitted. 

This is why symbol syncronization methods like Muller-Muller was developed.  I call this - for the sake of contrasting it with coarse OFDM symbol syncronization (achievable without interpolation and resampling) - fine-symbol-syncronization or smaller-than-time-interval-between-samples syncronizaiton.

Now, the question is, if such fine syncronization is important or not for OFDM. In OFDM there is the cyclic prefix which is much longer than the time interval between samples and there is also the FFT. It is not clear if in the case of OFDM the fine-symbol-syncronization is really neccessary. It might be that the guard interval+FFT makes it unneccessary.

One reason why fine-syncronization should not matter is that the purpose of the guard interval (cyclic prefix) is to make demodulation insensitive to channel response (multi path due to reflections). 

Is it important with OFDM to carry out fine, i.e. smaller-than-time-interval-between-samples syncronization ? 

EDIT: this book seems to discuss this question in Section "5.2.4 Synchronization Errors", it says that timing offset causes rotation - increasing in strength for sub-carriers further away from the center - need to read it more carefully.
Yes, fine timing synchronization is still very important for OFDM. As you know, an FFT is typically used to demodulate an OFDM signal once it's been properly synchronized and stripped of cyclic prefix. To see what would happen if you aren't finely synchronized in time, recall the time-shifting property of the DFT:

$$
x[n-l] \Leftrightarrow X[k] e^{j\frac{2\pi kl}{N}}
$$

That is, an arbitrary timing offset $l$ (which need not be an integer for this formula to be valid) causes a linear phase shift in each output bin that is proportional to its distance from zero frequency. 

So, your subcarriers near the center of the band will not be affected that much, but as you move outward, your demodulated signals will experience an increasing amount of phase rotation. That phase rotation is related to the unknown timing error $l$, and this value will likely vary slightly from symbol period to symbol period (assuming you don't have perfect symbol rate synchronization either). This all adds up to degraded performance.

The solution? You need fine symbol synchronization in OFDM, just like with any other digital modulation format.