I think I was able to narrow down the problem. When an (Alfred) action is triggered, the "smart synced" file is downloaded. This is no problem with small files, but with a big file (say, a 300 MB .psd) and a slow-ish internet connection, this can take a while. During this time, Alfred stays un-reactive (the window freezes / Alfred does not react), but the app does not actually crash … but if the file can't be downloaded (e.g. because of too slow internet connection), Alfred remains freezed.