Modify a dictionary which I am iterating through [duplicate]

You can’t modify a collection you’re iterating over. In this case, a much better solution would be to create a list of entries to remove by iterating over the dictionary, and then iterate over that list, removing the entries from the dictionary:

List<string> removals = new List<string>();                    
DateTime now = DateTime.Now;
foreach(BruteforceEntry be in Entries.Values)
{
    if (be.AddedTimeRemove <= now ||
        (be.Unbantime <= now && be.Unbantime.Day == DateTime.Now.Day))
    {
        removals.Add(be.IPAddress);
    }
}
foreach (string address in removals)
{
    Entries.Remove(address);
}

Note that if you’re using .NET 3.5, you could use a LINQ query to express the first part:

List<string> removals = (from be in Entries.Values
                         where be.AddedTimeRemove <= now ||
                               (be.Unbantime <= now && 
                                be.Unbantime.Day == DateTime.Now.Day)
                         select be.IPAddress).ToList();