Why Galaxy SII reports on not enough memory for new apps when I have several GBs available on flash storage?
The Samsung Galaxy SII is an older device which doesn't use the MTP protocol and so, applications' storage and internal storage is separated. Newer devices share the full internal storage seemingly.

If you in need of storage space for apps, you may look at some root options such as:


app2sd applications that enable you to transfer your applications to the internal storage
app2ext that enable you to extend your applications' partition
a ROM for your phone that change the partitions' size