Sony Reader PRS-T1 becomes unresponsive

My Sony Reader PRS-T1 based on Android 2.2 is acting funny. I had rooted it in the past but is now in the "original" state, upgraded with the latest official firmware. The problem is that many times, the unit becomes unresponsive. This happens after putting the unit on sleep and waking it again. The dictionary hangs, changing dictionaries hangs even worse and sometimes highlighting text as well (this makes the dictionary pop up. I managed to get logs and I found the following error messages, just after waking from sleepmode:

2013-01-22 17:15:06.391 W dalvikvm (2308:2308) [log_main    ] ERROR: Unable to find decl for native Lcom/sony/drbd/ebook/system/Identifier;.getProductName ()Ljava/lang/String;

2013-01-22 17:15:06.391 W dalvikvm (2308:2308) [log_main    ] ERROR: Unable to find decl for native Lcom/sony/drbd/ebook/system/Identifier;.getSkuName ()Ljava/lang/String;

2013-01-22 17:15:06.391 W dalvikvm (2308:2308) [log_main    ] ERROR: Unable to find decl for native Lcom/sony/drbd/ebook/system/Identifier;.getSerialNumber ()Ljava/lang/String;

2013-01-22 17:15:06.391 W dalvikvm (2308:2308) [log_main    ] ERROR: Unable to find decl for native Lcom/sony/drbd/ebook/system/Identifier;.getServiceId ()Ljava/lang/String;

2013-01-22 17:15:06.391 W dalvikvm (2308:2308) [log_main    ] ERROR: Unable to find decl for native Lcom/sony/drbd/ebook/system/Identifier;.getMacAddress ()Ljava/lang/String;

2013-01-22 17:15:06.391 W dalvikvm (2308:2308) [log_main    ] ERROR: Unable to find decl for native Lcom/sony/drbd/ebook/system/Identifier;.getPsn ()Ljava/lang/String;

2013-01-22 17:15:06.391 E JNIHelp (2308:2308) [log_main    ] RegisterNatives failed for 'com/sony/drbd/ebook/system/Identifier'

Any ideas on how to diagnose and restore my system?