TYPO3 Core

Every time I
1) log in or
2) refresh the TYPO3 (F5 or ctrl+r) or
3) go to the log module from the top bar

an error is logged into the system log module:

Core: Exception handler (WEB): Uncaught TYPO3 Exception: #1438871603: The given size is not a valid size, see Icon class for options | InvalidArgumentException thrown in file /Users/riccardo/TYPO3-dist/typo3_7_master/typo3/sysext/core/Classes/Imaging/Dimension.php in line 53. Requested URL: http://typo3.7.master.it:8888/typo3/index.php?ajaxID=%2Fajax%2Ficons&ajaxToken=9c99313ba6aa96a1e280be7e2a9fbd5e65597e93&requestedIcons=%5B%5B%22spinner-circle-light%22%2C%22small%22%2Cnull%2C%22default%22%5D%5D

The Chrome network inspector or error console reports a 500 error xhr

GET http://typo3.7.master.it:8888/typo3/index.php?ajaxID=%2Fajax%2Ficons&ajaxTo…5B%5B%22spinner-circle-light%22%2C%22small%22%2Cnull%2C%22default%22%5D%5D 500 (Internal Server Error) 

send    @    jquery-2.1.4.js:8630
jQuery.extend.ajax    @    jquery-2.1.4.js:8166
Icons.fetch    @    Icons.js?bust=dd389d0bd9cb74ddab5eb47ab5990d379173583d:103
Icons.getIcon    @    Icons.js?bust=dd389d0bd9cb74ddab5eb47ab5990d379173583d:44
SystemInformationMenu.updateMenu    @    SystemInformationMenu.js?bust=dd389d0bd9cb74ddab5eb47ab5990d379173583d:52
fire    @    jquery-2.1.4.js:3099
self.add    @    jquery-2.1.4.js:3145
jQuery.fn.ready    @    jquery-2.1.4.js:3378
jQuery.fn.init    @    jquery-2.1.4.js:2821
jQuery    @    jquery-2.1.4.js:76
(anonymous function)    @    SystemInformationMenu.js?bust=dd389d0bd9cb74ddab5eb47ab5990d379173583d:112
u.execCb    @    require.js:1
t.check    @    require.js:1
(anonymous function)    @    require.js:1
(anonymous function)    @    require.js:1
(anonymous function)    @    require.js:1
each    @    require.js:1
t.emit    @    require.js:1
t.check    @    require.js:1
t.enable    @    require.js:1
t.init    @    require.js:1
o    @    require.js:1
u.completeLoad    @    require.js:1
u.onScriptLoad    @    require.js:1