While trying to use the query editor in developer console.

"SELECT ID FROM ACCOUNT"

It throws an erorr saying "This session is not valid for use with the REST API". Any idea what excatly tthe issue here. Earlier it workerd fine.
The same problem when i click on "Open" dialog of developer console and select objects.

"CANNOT LOAD OBjects.This session is not valid for use with the REST API"

Here is my page header:
<apex:page standardController="Pro_Sale__c" cache="false" standardStylesheets="true" showHeader="false" applyBodyTag="false" renderAs="pdf">

Here is the part of my style that controls borders:
        table.gridtable {
            border-collapse: collapse;
            border: 0px;

         }

        th {
            border: 1px solid #000;
            padding: 3px;
            border-collapse : collapse;
            border-spacing: 0;

         }
         
        td {
            border: 1px solid #000;
            padding: 3px;
            border-collapse : collapse;
            border-spacing: 0;
         }
        

When I render as html, it looks fine, with all borders as 1px. However, when I render as pdf, some lines are double thickness (and there is no rhyme or reason why some are and some aren't, though the double thickness seem to be around th more than td). I've tried with standardstylesheets="true" and "false" and still get the same results.