In the meantime, you could try the following 2 potential work arounds
Workaround #1: add a call to drawnow before the call to exportgraphics. If it's a timing issue where the plot wasn't drawn when exportgraphics started to process it then this may help.
% ...
% ^^^ your original code to create the plot ^^^
filename='ExportTest.png';
% NEW: call drawnow to give the plot a chance to render
drawnow;
% your export call
exportgraphics(ax,filename);
Workaround #2: Rather than use gca, try creating the axes separately and then using that axes handle when plotting your data, adding the title, etc, and in the call to exportgraphics.
ax = axes;
t= [0 60 163 204 282 361 415 460 560 761];
U = [9.05 8.00 6.44 6.00 5.18 4.41 4.00 3.61 2.98 2.00];
plot(ax, t,U); % <<= use ax here
%title and stuff
title(ax, 'Grafic to export'); % <<= use ax here
xlabel(ax, 'X-factor'); % <<= use ax here
ylabel(ax, 'I don''t know Y '); % <<= use ax here
%export
%ax = gca; % <<= Don't use gca here
filename='ExportTest.png';
exportgraphics(ax,filename);
