If you get the following error on Oracle XE:

ORA-12520: TNS:listener could not find available handler for requested type of server

This is probably because the value for the processes parameter is too low.

For some reason, Oracle set the default value of the processes parameter on Oracle XE  to 40. If your application opens many process or  several queries are executed at the same time, the ORA-12520 could be displayed.

The solution for this issue is simple.  All you need to do is give the processes parameter an higher value.

open sql plus (All Programs -> Oracle Database 10g Express Edition -> Run SQL Command Line) and run the following command:
Alter system set processes=100 scope=spfile;

Restart the DB:
shutdown immediate
Startup

if needed, an higher number can be provided.