Still have the same error, code below for the VF page and controller.

<apex:page Controller="TimeEntry" tabStyle="Time_Slot__c">
    <apex:sectionHeader title="Multi Add"/>
        <apex:form >
    <apex:pageBlock title="Time Slot Quick Entry">
            <p>Fill in the rows below, when you choose a record type please ensure you select the corresponding type.</p>
            <p>&nbsp;</p>
            <apex:pageBlockButtons >
                <apex:commandButton value="Save" action="{!save}" rerender="error" />
            </apex:pageBlockButtons>
            <apex:pageBlockTable value="{!slots}" var="t" id="table" columnsWidth="50px, 50px, 50px, 100px, 100px, 50px, 300px">
                <apex:column headerValue="Record Type">
                    <apex:inputField value="{!t.recordtypeid}" required="true"/>
                </apex:column>
                <apex:column headerValue="Date">
                    <apex:inputField value="{!t.Date__c}" required="true"/>
                </apex:column>                  
                <apex:column headerValue="Type">
                    <apex:inputField value="{!t.Type__c}" required="true"/>
                </apex:column>
                <apex:column headerValue="Project">
                    <apex:inputField value="{!t.Project__c}"/>
                </apex:column> 
                <apex:column headerValue="Opportunity">
                    <apex:inputField value="{!t.Opportunity__c}"/>
                </apex:column>                                
                <apex:column headerValue="Time Spent">
                    <apex:inputField value="{!t.Time_Spent__c}" required="true"/>
                </apex:column>
                <apex:column headerValue="Description">
                    <apex:inputField value="{!t.Description__c}" rendered="true"/>
                </apex:column>   
            </apex:pageBlockTable>
    <apex:pageblockButtons location="bottom">
        <div style="text-align:right;margin-right:30px;font-weight:bold;">
            <apex:commandLink value="Add Row" action="{!addRow}" rerender="table,error" immediate="true" />
&nbsp;|&nbsp;&nbsp;
            <apex:commandLink value="Remove Row" action="{!removeRow}" rerender="table,error" immediate="true" />                
        </div>
    </apex:pageblockButtons>  
    </apex:pageBlock>
    </apex:form>
</apex:page>

public class TimeEntry {

    public List<Time_Slot__c> slots {get; set;}
    
    public TimeEntry(){        
        slots = new List<Time_Slot__c>();
        slots.add(new Time_Slot__c());
    }

    public void addrow() {
        slots.add(new Time_Slot__c());
    }
           
    public void removerow(){
        Integer i = slots.size();
        slots.remove(i-1);
    }
           
    public PageReference save() {
        insert slots;
        PageReference home = new PageReference('/a00/o');
        home.setRedirect(true);
        return home;
    }
}

Have you overridden the Tab link to your VF page? [ Setup-Create-Objects-Your Object-Standard Buttons and Links-Tab]

No I use a custom link from the home page to go to the VF Page.

I have just added a new tab using the VF with the same problem.

What is been selected in Setup-Create-Objects-Your Object-Standard Buttons and Links ?? Is the overridden column is checked?