System.Threading.Timer: A second operation was started on this context with - timer

Question

I have Console Application on .NET 5 which is Discord Bot.
While start the application i have the problem with this:
System.InvalidOperationException: A second operation was started on
this context before a previous operation completed. This is usually
caused by different threads concurrently using the same instance of
DbContext. For more information on how to avoid threading issues with
DbContext
And this is happens in Timer
This is in constructor:
_approveTimer = new Timer(async delegate
{
await CheckApproveAsync();
}, null, TimeSpan.Zero,
TimeSpan.FromSeconds(30));
public async Task CheckApproveAsync()
{
var pendingUsers = await _pendingUsers.GetAllAsync();
if (pendingUsers is null || !pendingUsers.Any())
return;
Program.Log.Debug($"Checking {pendingUsers.Count} pending users...");
foreach (var user in pendingUsers)
{
var expired = DateTime.UtcNow > user.ExpirationTime;
if (!expired) continue;
await _pendingUsers.DeleteWithDetachAsync(user);
IonicHelper.GetGuildUserById(_mainGuildId, user.UserId, out var sgUser);
try
{
var msg = await _messageService.GetMessageAsync("register-pending-expired", new FormatData(user.UserId));
await sgUser.SendIonicMessageAsync(msg);
}
catch (Exception ex)
{
await _serverHelper.SendLogAsync(_mainGuildId, "Error", $"{nameof(CheckApproveAsync)} - {ex.Message}");
}
}
var usersValidated = 0;
foreach (var user in pendingUsers)
{
RequestResult codeResult;
string code;
try
{
(codeResult, code) = await GetThirdPartyCodeByEncryptedSummonerIdAsync(user.Region, user.SummonerId);
}
catch (Exception)
{
continue;
}
if (code is null || codeResult != RequestResult.Success)
continue;
var sanitizedCode = new string(code.Where(char.IsLetterOrDigit).ToArray());
if (sanitizedCode != user.ConfirmationCode)
continue;
var (requestResult, summoner) = await GetSummonerByEncryptedPuuIdAsync(user.Region, user.PlayerUUID);
if (requestResult != RequestResult.Success)
{
await _pendingUsers.DeleteWithDetachAsync(user);
continue;
}
var (rankResult, rankData) = await GetLeaguePositionsByEncryptedSummonerIdAsync(user.Region, summoner.Id);
var soloqRank = GetRankModelFromEntry(rankData.FirstOrDefault(x => x.QueueType == "RANKED_SOLO_5x5"));
var summonerIcon = GetSummonerIconUrlById(summoner.ProfileIconId);
var lolData = new LeagueData
{
UserId = user.UserId,
SummonerRegion = user.Region,
PlayerUUID = summoner.Puuid,
AccountId = summoner.AccountId,
SummonerId = summoner.Id,
SummonerName = summoner.Name,
SummonerIcon = summonerIcon,
SummonerLevel = summoner.SummonerLevel,
SummonerRank = $"{soloqRank.Tier} {soloqRank.Rank}"
};
_ = IonicHelper.GetGuildUserById(_mainGuildId, user.UserId, out var sgUser);
await AssignRoleFromRankAsync(sgUser, soloqRank.Tier);
var data = await _leagueRepository.GetByIdAsync(user.UserId);
if (data == null)
{
await _leagueRepository.AddAsync(lolData);
}
usersValidated++;
user.SummonerName = lolData.SummonerName;
await PostValidateAsync(user);
}
Program.Log.Information($"{usersValidated} users validated.");
}
I read that it can be problem that if some method is not awaited, but i've checked it's all awaited. Which suggestions about this?