The error message "Your Payment Session has expired. To complete this transaction you will need to enter your information again."  is a little misleading.  It implies you are not typing fast enough – which is quite wrong. 

If you get "session timeout" errors on Paypal then it is most likely a "session cookie" related issue, ie, a cookie has been blocked, or a cookie has expired.  NB: A cookie is a temporary file created on your machine holding information required by the website.

So what are the fixes?
There are two ways that session cookies can be blocked, either or both might apply to you:

Firewalls
Firewalls (personal or corporate) can delete a session cookie very soon after it is created.
This means that the web site can make the session cookie but it is destroyed by the firewall.

This is known to apply to firewalls like EZ Firewall, Zone Alarm, Personal Firewall and BlackIce Defender among many others.

You may also have a firewall and not realize it. Many companies have firewalls installed and some ISP’s have a firewall system in place for their clients. Firewalls that affect session cookies will not only stop you from using Paypal but also many other web sites where you login to gain some level of access.

The fix: you need to change the settings on your firewall to allow cookies, or at least session cookies. See your firewall help or manual for details.

Web Browsers
Web browsers can also block session cookies, however this is easily fixed.

Note: it is important to close all browser windows after making the change suggested below and then open a new window for the new setting to be active.

Internet Explorer 6.0:
1. Start, Settings, Control Panel (or Click Start, then Control Panel for Windows XP)
2. Choose Internet Options
3. Select Privacy | Advanced
4. Check "Override automatic cookie handling"
5. Check "Always allow session cookies"
6. Click OK
7. Close all browser windows
8. Open a new browser window and go to our store.

For Internet Explorer 5.5 and below:
1. Click Start, Settings, Control Panel
2. Double-click Internet Options
3. then Security tab | Custom Level
4. Scroll to Allow per-session cookies
5. Select Enable
6. Click OK to save your changes
7. Close all browser windows
8. Open a new browser window and go to our store.

Mozilla Firefox users:
1. From the Tools menu, select "Options".
2. Click the Privacy icon.
3. Click the + sign to expand the Cookies section.
4. Select the "Allow sites to set cookies" option.
5. Click OK.

For Netscape 4.X users:
1. From the Edit menu, select "Preferences".
2. Click the + sign to expand the "Advanced" option.
3. Select the "Accept All Cookies" option.
4. Click "OK" to close the window.

The following is a representation of what PayPal say:

"The PayPal system is designed to drop your connection, or time out, after five minutes of inactivity.
If the system is timing out in less than five minutes, it may be caused by a lost (or dropped) secure connection, which is often a result of Internet congestion.

When Internet congestion occurs, secured web servers (the backbone of most websites) are affected the most. If the problem persists, disconnect and reconnect to your Internet connection (whether via a dial-up or DSL or cable connection).

You may also want to wait a few minutes before accessing the PayPal website. Also, make sure to clear your cache files periodically; otherwise, you may experience slow connection and closing times attempting to access or log out of the PayPal website.

To improve overall performance on the web, and specifically while visiting PayPal, routinely clear your cache by following these steps (if your web browser is not listed below, the best solutions are to check the browser's Help section or connect to its customer service center.)

For Internet Explorer:
Click the Start button.
Select Settings and then click Control Panel.
Double-click Internet (or Internet Options) button, and select the General tab.
Click the Settings button in the Temporary Internet Files section and choose View Files.
Click Edit and choose Select All.
Press the Delete key on your keyboard. If prompted to verify that you want to delete your cookies, click Yes.

For Netscape:
Computers store copies of frequently accessed web pages in the system's memory cache or disk cache.
This way, the computer doesn't have to download the web page each time you access it.
To change the size of the memory cache or clear it altogether (Windows and Unix only), follow these steps:

From the Edit menu, choose Preferences.
Open the Advanced category and click Cache.
To clear the Memory Cache immediately, click Clear Memory Cache.

For AOL:
Open the AOL account.
Click Settings on the toolbar, (My AOL for older versions).
Select Preferences.
Select the Internet Properties (WWW) link on the left-hand side.
In the middle of the Internet Options box is a section called Temporary Internet Files.
Click Delete Files. Place a check mark in the Delete All Offline Content box.
Click OK. "