Error at offset 2 of 954 bytes bootstrap.inc on line 130

After upgrading from 4.5.2 to 4.6 on a test site I started getting errors like the following:

Notice: unserialize(): Error at offset 2 of 954 bytes in c:\programmi\easyphp1-8\www\versus\includes\bootstrap.inc on line 130

warning: Cannot modify header information - headers already sent by (output started at c:\programmi\easyphp1-8\www\versus\includes\bootstrap.inc:130) in c:\programmi\easyphp1-8\www\versus\includes\common.inc on line 99.

warning: Cannot modify header information - headers already sent by (output started at c:\programmi\easyphp1-8\www\versus\includes\bootstrap.inc:130) in c:\programmi\easyphp1-8\www\versus\includes\common.inc on line 192.

What is that?