List of problems:
Could not connect; Could not connect to the Etherpad, Your connection is not making contact: The system was unable to connect to the Communication Device (Modem/Etherpad). Check that the Comminication Device is setup correctly. You can get more information in the first section of this wiki here
Communication error: The system did connect to the Communication Device, and there was communication, but the communication dropped before the system was done reading the Meter. This could be due to:
Bad Signal (very likely)
Not enough data on Prepaid SIM (likely)
Short on the communication line. (unlikely) Typically with RS485, if the RX and TX line touches, everything that is transmitted is also received, making it look like there is proper comms, but there is not.
Wrong COM port settings (unlikely). Communications using RS232 or 485 need to be programmed with a BAUD rate and Framing. Wrong settings might result in garbled comms.
Communication error, no answer back: The system did connect to the Communication Device, but there was no comms. This could be due to:
Not enough data on Prepaid SIM (extremely likely)
Bad Signal (very likely)
Break in the communication line (unlikely)
Wrong COM port settings (unlikely). Communications using RS232 or 485 need to be programmed with a BAUD rate and Framing. Wrong settings might result in no comms.
Connection reset: Happens when the Communication Device restarts.
CT discrepancy. Please fix first.: Typically the meter gets programmed with the CT ratio. The CT ratio also gets recorded in PNPSCADA when you add the meter. The CT ratio in PNPSCADA should match the one in the meter.
Meter not Found: A connection was made to the Communication Device but the meter did not respond. 
Accumulator not found: same as Meter not Found
Wrong Meter At Address: The meter serial number that was read from the meter, and the serial number that you entered into the system differs. For Elster A1140/A1700 and AS230 the Multidrop Address in PNPSCADA and on the Meter need to match. You can use Elster's SMARTset to get the correct multidrop address. Go to Edit->Meter Details to change the Multidrop Address in PNPSCADA.
Output Problem: You will need to contact SDG (010 003 1015) for this.
unlicensed: there are no licenses to read this meter. Please contact SDG to resolve this issue.
Invalid Meter Password: You can use the OEM software (SMARTset/MAP120/HexingView etc) to set the password on meters. You then need to enter that password in Edit->Meter Details->Password so that PNPSCADA can log into the meter.
Could not find RTU at address; PLC meter not found; Could not find meter at address: The Communication Device is connected to the server, but the Meter is not answering back. See Communication Error, no answer back.
Spike: PNPSCADA calculates the maximum kW that can go through this meter with VTprim * Breaker Size * Number of Phases. So if you add a 3 phase meter with a 400/400 VT and a breaker size of 100A, that means 400*100*3=120000 or 120kW maximum. If a values is read from the meter that is more than 10 times the maximum (in this case more than 1200kW) a spike is reported and the meter will not be read further. Typically you need to enter the correct breaker size.
Still Waiting:
Component Meter Not Read In:
RF failure:
Not InUse: For Kamstrup Meters it means no concentrator has picked up the meter yet. The problem is between the meter and the Kamstrup server.

No Meter Registers read  This means that although the Billing Registers checkbox has been ticked in Advanced Meter Settings, no Meter Registers has been read for this meter yet. 

Meter Registers times wrong This means that Meter Register/s have been read from the Meter, but they are not on a profile period or billing period boundary, so they cannot be used for billing.

No B.Registers recognized This means that although Meter Registers have been read, they are not recognized according to the TOU Calender register descriptors of a TOU Calendar configured for this meter on Advanced Meter Settings, so they cannot be associated to TOU Billing Registers to be used for billing.

B.Registers times wrong This means that although Meter Registers was read and they were associated with TOU Billing Registers, they still cannot be used for billing because they are not on a profile period or billing period boundary.

Invalid Password This means that the login on your meter failed, probably due to a wrong password for the amount of access requested. If a Landis & Gyr meter, try removing the password to use the Public Login. Note, if you use the Public Login, some values and functions might be unavailable, for instance you'd typically not be able to set the meter time.

This list is a work in progress, and might be slightly out of date.

In the case where a meter read is not even attempted by the system (i.e. View->Meter Read Logs are empty), check current error streak under Edit->Advanced Meter Settings.

You can log a fault against a meter by clicking on None yet in the Follow up column. This will take you to a screen where you can assigned a technician to a meter and log any faults communicated by the technician.

If the Problem Meter List suddenly do not show any meters (and there should be meters), it might be because one of your meters were marked as Commissioned under Edit->Meter Details.
When no meters have the Commissioned Flag ticked, the system ignores the Commissioned Flag. If one or more meters have it ticked, then only meters that are commissioned are shown on the Problem Meter List.
You can get a list of which meters are commissioned and uncommissioned under Reports->Various->Meters Where and Read.
Click toggle, to show both commissioned and uncommissioned meters.

To get a list of which meters did read in, go to Reports->Various->Meters Date

The download at the bottom will give a spreadsheet that contains many more columns if you require more information.